Search Results (54)

Significant differences in hydrogen adsorption on amorphous and crystalline gold nanoparticles deposited on highly oriented pyrolytic graphite (HOPG) were revealed. Crystalline nanoparticles were synthesized via the impregnation–precipitation method followed by annealing at 700 K, whereas amorphous ones were obtained using the laser electrodispersion method. The morphology and electronic structure of single nanoparticles were investigated with high spatial resolution using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum both before and after exposure to molecular hydrogen at doses of 400–6000 L. Experiments performed at room temperature showed that the surface coverage by the adsorbate in both cases begins at the Au-HOPG interface, spreads towards the center of the particle, and corresponds to the island growth model. However, amorphous nanoparticles have fewer growth sites at the periphery compared to crystalline ones. The local electronic structure of amorphous nanoparticles is more inhomogeneous compared to crystalline ones, demonstrating variation across different points on the nanoparticle surface. It was shown that dissociative chemisorption of hydrogen takes place on amorphous gold nanoparticles with a size of 4–6 nm. Chemisorption is completely inhibited when the nanoparticle size is reduced to 2 nm or less. Full article

The intensification of industrialization and increasing energy consumption have led to elevated emissions of hazardous gases such as NO, NO₂, and SO₂, making their efficient capture and removal crucial for environmental remediation. In this work, first-principles calculations were employed to systematically investigate the adsorption behavior of these gases on single-atom-decorated (Sc, Ti, and V) 1T-ZrS₂ monolayers. The results indicate that the transition metal atoms preferentially occupy the hexagonal hollow sites of ZrS₂, forming an approximately octahedral coordination field and exhibiting characteristic d-orbital splitting. During gas adsorption, the decorated systems exhibit pronounced metal-to-adsorbate charge donation and strong d-p hybridization, indicative of strong chemisorption. Notably, Ti-ZrS₂ exhibits the strongest adsorption toward NO₂, inducing partial molecular dissociation and suggesting catalytic activity, whereas Sc- and V-decorated systems predominantly maintain molecular adsorption. Recovery time calculations indicate that the adsorption processes are comparatively stable, making these systems suitable for gas capture and pollution abatement. Overall, single-atom decoration provides an effective strategy to modulate the electronic structure and gas interactions of ZrS₂, highlighting its potential as an efficient gas scavenger for NO, NO₂, and SO₂. Full article

In chemical vapor deposition (CVD)-mediated graphene growth, copper foil serves as both a catalyst for methane decomposition and as a substrate for graphene nucleation and growth. Due to the low solubility of carbon in copper and the ease of transferring graphene from its surface, copper—particularly the Cu(111) facet—is widely favored for high-quality, monolayer graphene synthesis. In this article, the thermodynamic processes involved in methane dissociation and graphene nucleation on the Cu(111) surface were investigated using density functional theory (DFT). Molecular dynamics simulations were performed for structural optimization and to evaluate the reaction energies. Additionally, the average adsorption energies (ΔEad) of carbon clusters with varying atomic numbers on the Cu(111) surface were calculated. The graphene growth process was further modeled using the kinetic Monte Carlo (KMC) method to simulate carbon atom migration and nucleation dynamics. Thermodynamic analysis based on equilibrium component data was conducted to examine the influence of key operational parameters—temperature, pressure, and the CH₄/H₂ partial pressure ratio—on the graphene deposition rate. Full article

The conversion of CO₂ into light olefins over bifunctional catalysts is a promising route for producing high-value-added products. This approach not only mitigates excessive CO₂ emissions but also reduces the chemical industry’s reliance on fossil fuels. Among bifunctional catalysts, ZnZrO_x is widely used due to its favorable oxide composition. In this work, ZnZrO_x solid solution was synthesized by calcining an MOF precursor, resulting in a large specific surface area and a small particle size. Characterization studies revealed that ZnZrO_x prepared via MOF calcination exhibited an enhanced CO₂ activation and H₂ dissociation capacity compared to that synthesized using the co-precipitation method. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that CO₂ adsorption on ZnZrO_x led to the formation of carbonate species, while HCOO* and CH₃O* intermediates were generated upon exposure to the reaction gas. When ZnZrO_x was combined with SAPO-34 molecular sieves under reaction conditions of 380 °C, 3 MPa, and 6000 mL·g_cat⁻¹·h⁻¹, the CO₂ conversion reached 34.37%, with a light olefin yield of 15.13%, demonstrating a superior catalytic performance compared to that of the co-precipitation method. Full article

The search for alternative sources of, and substitutes for, chemicals derived from fossil-based feedstocks encourages studies of heterogeneous catalysts to increase the feasibility of sustainable production of biomass derivatives, such as γ-valerolactone, among others. In this context, first, the performance of a titania-supported nickel catalyst (a non-noble catalyst) was evaluated in the reaction of hydrogenation of levulinic acid to γ-valerolactone in water using molecular hydrogen. The methods used included the synthesis of titania via the solgel method and nickel deposition by deposition–precipitation via removal of the complexing agent. The nickel was activated in a flow of hydrogen; the temperature of reduction and the calcination step were investigated with experiments at reaction conditions to study the catalyst’s stability. Then, after a statistical evaluation of several proposed kinetic models, the kinetics of the reaction was found to be best represented by a model obtained considering that the reaction over the surface was the determinant step, followed by the non-dissociative adsorption of hydrogen and the competitive adsorption among hydrogen, levulinic acid, and γ-valerolactone. With that model, the activation energy of the levulinic acid to 4-hydroxypentanoic acid step was (47.0 ± 1.2) kJ mol⁻¹, since the determinant step was the hydrogenation reaction of the levulinic acid to 4-hydroxypentanoic acid. It was also concluded that the catalyst prepared was stable, active, and selective to γ-valerolactone. Full article

The adsorption and reaction of nitric oxide (NO) molecules on the surface of the model-supported metal/oxide system, consisting of Ni nanoparticles deposited on α-Al₂O₃ (0001) in ultra-high vacuum, have been studied using in situ surface-sensitive techniques and density functional theory (DFT) calculations. As a combination of X-ray and Auger electron spectroscopy (XPS, AES), Fourier-transform infrared (FTIR) spectroscopy, and temperature-programmed desorption (TPD) techniques reveals, there is a threshold of Ni particle mean size (<d>) of c.a. 2 nm, differentiating the electron state of adsorbed NO molecules and their reaction. The main feature of Ni particles normally not exceeding 2 nm is that the NO adsorbs in the form of (NO)₂ dimers, whereas, for larger particles, the NO molecules adsorb in the form of monomers, usually characteristic for the bulk Ni substrate. This difference is demonstrated to be the main reason for the different reaction of NO molecules on the surface of Ni/alumina. The striking feature is that, in the case of ultra-small Ni particles (<d> ≤ 2 nm), the nitrous oxide (N₂O) molecules are formed upon heating as a result of the NO self-reduction mechanism, which are otherwise not formed in the case of larger Ni particles. According to DFT results, this is due to the significant synergistic impact of NO co-adsorption on the neighboring NO dissociation reaction over ultra-small Ni particles, mediated by the metal/oxide perimeter interface. The observed molecular conversion effects offer an opportunity to tune the catalytic selectivity of this and related metal/oxide systems via varying the supported metal particle size. Full article

The geometrical structure, stability, electronic properties, and hydrogen storage capabilities of a titanium-doped B₇ cluster was calculated using density functional theory computations. The results show that the TiB₇ cluster is predicted to be stable under near-ambient conditions based on an ab initio molecular dynamic simulation. The transition state analysis found that the H₂ molecule can dissociate on the TIB₇ cluster surface to form a hydride cluster. The Ti atom within the TiB₇ cluster demonstrates an impressive capacity to adsorb up to five H₂ molecules, achieving a peak hydrogen storage mass fraction of 7.5%. It is worth noting that the average adsorption energy of H₂ molecules is 0.27–0.32 eV, which shows that these configurations are suited for reversible hydrogen storage under mild temperature and pressure regimes. In addition, calculations found that both polarization and hybridization mechanisms play pivotal roles in facilitating the adsorption of H₂ molecules onto the TiB₇ cluster. Our research results show that the TiB₇ cluster has potential for hydrogen storage applications under near-ambient conditions. Full article

Environmentally friendly nanoporous gels are tailor-designed and employed in the adsorption of toxic organic pollutants in wastewater. To ensure the maximum adsorption of the contaminant molecules by the gels, molecular modeling techniques were used to evaluate the binding affinity between the toxic organic contaminants such as methylene blue (MB) and Congo red (CR) and various biopolymers. To generate nanopores in the matrix of the polymeric gels, salt crystals were used as porogen. The pores were then used to accommodate catalytic nickel (Ni⁰) nanoparticles. Under UV irradiation, the nanoparticles demonstrated the effective adsorption and photocatalytic degradation of both the methylene blue and Congo red dyes, achieving removal efficiencies of up to 90% for MB and 80% for CR. The thermodynamic analysis suggested a spontaneous endothermic dissociative adsorption mechanism, which implies the oxidative catalytic degradation of the dyes. The kinetic modeling suggested a pseudo-second-order model, while the model for intra-particle diffusion revealed that Congo red diffuses faster than methylene blue. MB adsorption followed a Langmuir isotherm, while CR adsorption followed a linear isotherm. The results confirm that dye molecules initially undergo physisorption and subsequent dissociative adsorption. The products of the catalytic degradation of methylene blue continue to be absorbed on the surface of the nanoparticles, while those of Congo red switch to preferential desorption. Full article

There is no doubt that when solid surfaces are modified, the functional groups and atoms directly bonded to solid atoms play a major role in adsorption interactions with molecules or resins. In this study, the adhesion and dissociation between epoxy resin and molecules containing Si atoms were analyzed. The analysis, conducted in contact with the solid surface of silicon, utilized quantum chemical calculations based on a molecular model. We compared some Si-containing molecular models to test quantum chemical calculations that contribute to the study of adhesion and dissociation between epoxy resins and solid surfaces somehow other than simple potential energy curve calculations. The AFIR (artificial force induced reaction) method, implemented in the GRRM (global reaction route mapping) program, was employed to separate an epoxy resin model molecule and three types of silicon compounds (Si(CH₃)₂(OH)₂, Si(CH₃)₄, and (CH₃)₂SiF₂) in three directions, determining their minimum dissociation energy when changing the applied energy by 2.5 kJ/mol. In systems with weak hydrogen bonds, such as Si(CH₃)₄ or (CH₃)₂SiF₂, the energy required for dissociation was not large; however, in systems with strong hydrogen bonds, such as Si(CH₃)₂(OH)₂, dissociation was more difficult in the vertical direction. Although anisotropy due to hydroxyl groups was calculated in the horizontal direction, dissociation remained relatively easy. Full article

To investigate hydrogen behaviors in the high-entropy alloy AlCrTiNiV, density functional theory and transition state theory were used to explore the molecular $H_2$ absorption and dissociation and the atomic H adsorption, diffusion, and penetration progress. The $H_2$ molecule, where the H-H band is parallel to the surface layer, is more inclined to absorb on the top site of the Ti atom site of first atomic layer on the AlCrTiNiV surface, then diffuse into the hollow sites, through the bridge site, after dissociating into two H atoms. Atomic H is more likely to be absorbed on the hollow site. The absorption capacity for atomic H on the surface tends to decline with the increase in H coverage. By calculating the energy barriers of atomic H penetration in AlCrTiNiV, it was indicated that lattice distortion may be one important factor that impacts the permeation rate of hydrogen. Our theory research suggests that high-entropy alloys have potential for use as a hydrogen resistant coating material. Full article

Crystalline poly-para-xylylene (parylene) has the potential for use as a protective membrane to delay the nucleation of explosives by separating the explosives and their decomposition products to decrease the explosive sensitivity. Here, molecular dynamics (MD) and density functional theory (DFT) techniques were used to calculate the dissociative adsorption configurations of 1,1-diamino-2,2-dinitroethylene (FOX-7) on (001)- and (101)-oriented crystalline parylene membranes. Based on the results of the calculations, this work demonstrates that the -NO₂–π electrostatic interactions are the dominant passivation mechanism of FOX-7 on these oriented surfaces. FOX-7 can dissociatively adsorb on oriented parylene membranes due to the interactions between the LUMO of the toluene (or methyl) groups on parylene and the HOMO of the -NO₂ (or -NH₂) groups on FOX-7. The formation of a new intermolecular H-bond with the ONO group leads to FOX-7 decomposition via intramolecular C-NO₂ bond fission and nitro-to-nitrite rearrangement. The most likely adsorption configurations are described in terms of the decomposition products, surface active groups of parylene, binding behaviors, and N charge transfer. Importantly, the (001)-oriented parylene AF8 membrane is promising for use as a protective membrane to passivate the high-energy -NO₂ bonds during the dissociative adsorption of FOX-7. This study offers a new perspective on the development of protective membranes for explosives. Full article

We used ab initio molecular dynamics simulations to compare the activation of 2-propanol on the low index Co₃O₄ (111), (110) and (001) surfaces in dry conditions. The thermal and surface assisted decomposition of a film of 2-propanol to 2-propoxide on the B-termination of each surface was monitored and analyzed. The investigations suggest an activity order of Co₃O₄ (111) > (110) > (001). On all surfaces, the Co³⁺ serve as adsorption sites. On the B-terminated (111) surface, full dissociation of all 2-propanol molecules at the interface is observed, accompanied by a Mars-van Krevelen-type mechanism upon pre-hydroxylation of the surface. The active regions show Co³⁺–O_2-propoxide–Co²⁺ bridges where the coordinatively unsaturated Co²⁺ ions also participate in the adsorption and decomposition of 2-propanol. On the (110) surface, 2-propanol dissociation is driven by temperature, which activates the two-fold coordinatively unsaturated surface oxygens. The (001) surface on which almost no dissociation occurs is the least active. No formation of acetone is observed in the simulations conditions on all surfaces. Full article

Spinel ferrites, ZnFe₂O₄, MnFe₂O₄, and ZnMnFe₂O₄, were synthesized using the sol–gel method and thoroughly investigated for their potential as catalytic and magnetic materials. Experiments unveiled that ZnMnFe₂O₄ exhibited excellent catalytic and magnetic properties, whereas the Density Functional Theory (DFT) calculations provided insight into the excellent performance of ZnMnFe₂O₄ compared with ZnFe₂O₄ and MnFe₂O₄. The catalytic efficiencies of the synthesized spinel ferrites were evaluated against a model reaction, i.e., the gas-phase oxidation of octanol to a corresponding aldehyde, utilizing molecular oxygen as an oxidant. The results indicated that the order of catalytic activity was ZnMnFe₂O₄ > MnFe₂O₄ > ZnFe₂O₄. The reaction was found to follow Langmuir Hinshelwood’s mechanism for dissociative adsorption of molecular oxygen. Owing to their superb catalytic and magnetic properties, mixed ferrites can be extended to a variety of organic transformation reactions. Full article

Dispersants, serving as an essential raw material in the formulation of coal water slurry, offer an economical and convenient solution for enhancing slurry concentration, thus stimulating significant interest in the development of novel and efficient dispersants. This paper intends to illuminate the evolution of dispersants by examining both the traditional and the newly conceived types and elaborating on their respective mechanisms of influence on slurry performance. Dispersants can be classified into anionic, cationic, amphoteric, and non-ionic types based on their dissociation properties. They can be produced by modifying either natural or synthetic products. The molecular structure of a dispersant allows for further categorization into one-dimensional, two-dimensional, or three-dimensional structure dispersants. This document succinctly outlines dispersants derived from natural products, three-dimensional structure dispersants, common anionic dispersants such as lignin and naphthalene, and amphoteric and non-ionic dispersants. Subsequently, the adsorption mechanism of dispersants, governed by either electrostatic attraction or functional group effects, is elucidated. The three mechanisms through which dispersants alter the surface properties of coal, namely the wetting dispersion effect, electrostatic repulsion effect, and steric hindrance effect, are also explained. The paper concludes with an exploration of the challenges and emerging trends in the domain of dispersants. Full article

In this study, we considered the structural stability, electronic properties, and phonon dispersion of the cubic (c-ZrO₂), tetragonal (t-ZrO₂), and monoclinic (m-ZrO₂) phases of ZrO₂. We found that the monoclinic phase of zirconium dioxide is the most stable among the three phases in terms of total energy, lowest enthalpy, highest entropy, and other thermodynamic properties. The smallest negative modes were found for m-ZrO₂. Our analysis of the electronic properties showed that during the m–t phase transformation of ZrO₂, the Fermi level first shifts by 0.125 eV toward higher energies, and then decreases by 0.08 eV in the t–c cross-section. The band gaps for c-ZrO₂, t-ZrO₂, and m-ZrO₂ are 5.140 eV, 5.898 eV, and 5.288 eV, respectively. Calculations based on the analysis of the influence of doping 3.23, 6.67, 10.35, and 16.15 mol. %Y₂O₃ onto the m-ZrO₂ structure showed that the enthalpy of m-YSZ decreases linearly, which accompanies the further stabilization of monoclinic ZrO₂ and an increase in its defectiveness. A doping-induced and concentration-dependent phase transition in ZrO₂ under the influence of Y₂O₃ was discovered, due to which the position of the Fermi level changes and the energy gap decreases. It has been established that the main contribution to the formation of the conduction band is made by the p-states of electrons, not only for pure systems, but also those doped with Y₂O₃. The t-ZrO₂ (101) and t-YSZ (101) surface models were selected as optimal surfaces for water adsorption based on a comparison of their surface energies. An analysis of the mechanism of water adsorption on the surface of t-ZrO₂ (101) and t-YSZ (101) showed that H₂O on unstabilized t-ZrO₂ (101) is adsorbed dissociatively with an energy of −1.22 eV, as well as by the method of molecular chemisorption with an energy of −0.69 eV and the formation of a hydrogen bond with a bond length of 1.01 Å. In the case of t-YSZ (101), water is molecularly adsorbed onto the surface with an energy of −1.84 eV. Dissociative adsorption of water occurs at an energy of −1.23 eV, near the yttrium atom. The results show that ab initio approaches are able to describe the mechanism of doping-induced phase transitions in (ZrO₂+Y₂O₃)-like systems, based on which it can be assumed that DFT calculations can also flawlessly evaluate other physical and chemical properties of YSZ, which have not yet been studied quantum chemical research. The obtained results complement the database of research works carried out in the field of the application of biocompatible zirconium dioxide crystals and ceramics in green energy generation, and can be used in designing humidity-to-electricity converters and in creating solid oxide fuel cells based on ZrO₂. Full article