Monolayer Coverage Research Articles

A site distance dependence of product selectivity in n-hexane dehydrogenation over alumina-supported chromium catalyst

Although CrOx/Al2O3 has been widely applied for light alkanes dehydrogenation, there is insufficient understanding on the dehydrogenation of long-chain alkanes that no solutions to the poor selectivity to mono-olefins and high yield of aromatics. Herein, dehydrogenation of n-hexane was studied on a series of CrOx/γ-Al2O3 catalysts with different densities of Cr sites, and it finds that the dehydrogenation performance is closely related to the distance between Cr sites. With densely populated Cr sites, multiple H atoms of n-hexane are adsorbed on the CrOx/γ-Al2O3 simultaneously (i.e., multi-point adsorption), promoting the formation of n-hexadiene, n-hexatriene and the resultant benzene and/or coke. Therefore, when the Cr loading exceeds the mono-layer coverage (5 wt% Cr loadings), the selectivities to n-hexenes and benzene are kept constant (about 23 wt% and 68 wt%, respectively). While decrease of Cr content in the mono-layer regime leads to a significant decrease in both the distance between Cr sites and the number of adsorbed H atoms, resulting in improved selectivity to n-hexenes. The CrOx@γ-Al2O3 catalyst with 3.6 Cr atoms/nm2 prepared by the equilibrium adsorption method (and the complexation of CrIII and 3,5-dimethylpyridine), exhibits 89 wt% selectivity to n-hexenes (<2 wt% for benzene). This work provides theoretical guidance for the precise design and synthesis of the high-performance CrOx-based catalyst for the dehydrogenation of alkanes, especially for long-chain alkanes.

Adsorption Behaviors and Mechanism of Phenol and Catechol in Wastewater by Magnetic Graphene Oxides: A Comprehensive Study Based on Adsorption Experiments, Mathematical Models, and Molecular Simulations.

This study provides a comprehensive analysis of the adsorption behaviors and mechanisms of phenol and catechol on magnetic graphene oxide (MGO) nanocomposites based on adsorption experiments, mathematical models, and molecular simulations. Through systematic experiments, the influence of various parameters, including contact time, pH conditions, and ionic strength, on the adsorption efficacy was comprehensively evaluated. The optimal contact time for adsorption was identified as 60 min, with the observation that an increase in inorganic salt concentration adversely affected the MGOs' adsorption capacity for both phenol and catechol. Specifically, MGOs exhibited a superior adsorption performance under mildly acidic conditions. The adsorption isotherm was well represented by the Langmuir model, suggesting monolayer coverage and finite adsorption sites for both pollutants. In terms of adsorption kinetics, a pseudo-first-order kinetic model was the most suitable for describing phenol adsorption, while catechol adsorption conformed more closely to a pseudo-second-order model, indicating distinct adsorption processes for these two similar compounds. Furthermore, this research utilized quantum chemical calculations to decipher the interaction mechanisms at the molecular level. Such calculations provided both a visual representation and a quantitative analysis of the interactions, elucidating the underlying physical and chemical forces governing the adsorption phenomena. The findings could not only offer crucial insights for the treatment of coal industrial wastewater containing phenolic compounds with bridging macroscopic observations with microscopic theoretical explanations but also advance the understanding of material-pollutant interactions in aqueous environments.

Effective Removal of Carcinogenic Azo Dye from Water Using Zea mays-Derived Mesoporous Activated Carbon.

Addressing industrial wastewater treatment challenges and removing hazardous organic pollutants, such as carcinogenic methyl orange (MO) and azo dyes, is a pressing concern. This study explores the use of the Zea mays envelope, an agricultural waste product, to produce Z. mays activated carbon (ZMAC) through the chemical activation of maize envelopes with phosphoric acid. Various analytical techniques, including FTIR, XRD, TGA, DSC, and SEM, characterize ZMAC. Results show that ZMAC exhibits an impressive monolayer adsorption capacity of 66.2 mg/g for MO. The Langmuir isotherm model fits the experimental data well, indicating monolayer coverage of the MO on the ZMAC surface. The pH-sensitive adsorption process demonstrates an optimal removal efficiency at pH 4. ZMAC follows the pseudo-second-order kinetic model, and diffusion rate constant analysis identifies three consecutive stages in the adsorption process. Moreover, the uptake of MO ions by ZMAC is identified as an exothermic and spontaneous process. Reusability tests demonstrate efficient regeneration of ZMAC up to five times with 1 mL of 2 M HNO3 in each cycle, without sorbent mass loss. Thermodynamic analysis shows an increase in the uptake capacity from 66.2 to 73.2 mg/g with temperature elevation. This study offers practical solutions for industrial wastewater treatment challenges, providing an environmentally sustainable and effective approach to mitigate the risks associated with hazardous organic pollutants.

Operando study of HfO2 atomic layer deposition on partially hydroxylated Si(111)

The introduction of atomic layer deposition (ALD), to the microelectronics industry has introduced a large number of new possible materials able to be deposited in layers with atomic thickness control. One such material is the high-κ oxide HfO2; thermally stable and ultrathin HfO2 films deposited by ALD are a significant contender to replace SiO2 as the gate oxide in capacitor applications. We present a mechanistic study of the first deposition cycle of HfO2 on the Si(111) surface using tetrakis(dimethylamido) hafnium (TDMAHf) and water as precursors using operando ambient pressure x-ray photoelectron spectroscopy. Here, we show that the hydroxylation of the clean Si(111) surface by residual water vapor, resulting in a 0.3 monolayer coverage of hydroxyls, leads to instantaneous full surface coverage of TDMAHf. The change in the atomic ratio of Hf to C/N found during the first deposition half-cycle, however, does not match the assumed immediate ligand loss through reaction with surface hydroxyls. One would expect an immediate loss of ligands, indicated by a Hf:N ratio of approximately 1:3 as TDMAHf deposits onto the surface; however, a Hf:N ratio of 1:3.6 is observed. The partial hydroxylation on the Si(111) surface leads to binding through the TDMAHf ligand N atoms resulting in both N and CH3 being found remaining on the surface post water half-cycle. Although there is evidence of ligand exchange reactions occurring at Si–OH sites, it also seems that N binding can occur on bare Si, highlighting the complexity of the substrate/precursor reaction even when hydroxyls are present. Moreover, the initial low coverage of Si–OH/Si–H appears to severely limit the amount of Hf deposited, which we hypothesize is due to the specific geometry of the initial arrangement of Si–OH/Si–H on the rest- and adatoms.

Utilization of water hyacinth (Eichhornia crassipes) biomass as eco-friendly sorbent for petroleum oil spill cleanup

Abstract This study focuses on the removal of spent engine oil (SEO) spill from the water surface using water hyacinth biomass (WHB)-based sorbents. The raw WHB was modified using extra virgin coconut oil (mainly consisting of lauric acid) to enhance the hydrophobicity and floating ability. With varying amounts of coconut oil and solvent, six diverse types of modified water hyacinth biomass (MWHB) were prepared. Among these MWHBs, an equal proportion of coconut oil and raw WHB with 10% methanol solution exhibited the highest removal of SEO reaching 96%. Various sorption kinetics and isotherm models were examined to understand the SEO sorption process on MWHB. The pseudo-second-order kinetics (R2 0.999) and both the Langmuir and Freundlich isotherm models (R2 0.992 and R2 0.999, respectively) were found to be the best-fitting models. These findings indicated a chemisorption mechanism involving the initial monolayer coverage of SEO molecules on the MWHB surface followed by the development of multilayers. The MWHB achieved a maximum sorption capacity of 4.75 g/g within 60 min. Furthermore, the reusability tests showed that MWHB maintained a sorption capacity of over 90% even after the third sorption–desorption cycle.

Multifunctional magnetic composite: MgAl-layered double hydroxide, activated carbon and magnetite on removing both different charged-dyes

The activated carbon (AC) prepared from the shells of coconut and palm, and MgAl-layered double hydroxide (MgAl-LDH) were performed as the adsorbent for removing cationic (methylene blue) and anionic (orange II) dyes in water. The precursors of AC, MgAl-LDH and Fe3O4 were mechanically mixed to provide the magnetic composites, which were investigated by XRD, FT-IR, SEM, elemental mapping, BET, VSM, zeta potential and UV–VIS. The composites showed the characteristics and the synergistic effects of AC, MgAl-LDH and Fe3O4 with large specific surface area (320 – 330 m2/g) due to AC and MgAl-LDH, and high magnetization (12.5 – 14.4 emu/g) due to Fe3O4. The composite adsorbents revealed the bifunctional active-surface on removing both methylene blue (qm = 568 – 625 mg/g) and orange II (qm = 518 – 550 mg/g) based on monolayer coverage, and pseudo-second order reaction and spontaneous process. After 5th cycle, the adsorption capacity was reduced just 6.0 – 9.3 % with high performance on the structure and magnetism.

Monolayer calibration of endofullerenes with x-ray absorption from implanted keV ion doses

X-ray absorption spectroscopy (XAS) has the highest sensitivity for chemical element detection on surfaces. With this approach, small amounts of lanthanide-containing endofullerene molecules (Ho3N@C80) have been measured by total electron yield at a low flux bending magnet beamline. The monolayer coverage is calibrated by extrapolating the signals of constant doses (3×1014 cm−2) of Ho ions implanted into SiO2 with energies between 2 and 115 keV. At room temperature, the Ho XAS spectra of the molecules and implanted ions indicate trivalent but not identical Ho ground states. Still, this approach demonstrates a way for calibration of small coverages of molecules containing open core-shell elements.

Co-Self-Assembled Monolayers Modified NiOx for Stable Inverted Perovskite Solar Cells.

[4-(3,6-dimethyl-9H-carbazol-9yl)butyl]phosphonic acid (Me-4PACz) self-assembled molecules (SAM) are an effective method to solve the problem of the buried interface of NiOx in inverted perovskite solar cells (PSCs). However, the Me-4PACz end group (carbazole core) cannot forcefully passivate defects at the bottom of the perovskite film. Here, a Co-SAM strategy is employed to modify the buried interface of PSCs. Me-4PACz is doped with phosphorylcholine chloride (PC) to form a Co-SAM to improve the monolayer coverage and reduce leakage current. The phosphate group and chloride ions (Cl-) in PC can inhibit NiOx surface defects. Meantime, the quaternary ammonium ions and Cl- in PC can fill organic cations and halogen vacancies in the perovskite film to enable defects passivation. Moreover, Co-SAM can promote the growth of perovskite crystals, collaboratively solve the problem of buried defects, suppress nonradiative recombination, accelerate carrier transmission, and relieve the residual stress of the perovskite film. Consequently, the Co-SAM modified devices show power conversion efficiencies as high as 25.09% as well as excellent device stability with 93% initial efficiency after 1000 h of operation under one-sun illumination. This work demonstrates the novel approach for enhancing the performance and stability of PSCs by modifying Co-SAM on NiOx.

Tungsten oxide nanomaterial interactions below monolayer coverage

A tungsten oxide load at submonolayer coverage of alumina renders highly dispersed dimeric polytungstate (octahedral and distorted octahedral coordination) and isolated monotungstate (tetrahedral) species. The polytungstate/monotungstate proportion increases when approaching monolayer. Crystalline WO3 nanoparticles emerge at higher loads. All components display hexavalent oxidation; yet, tungsten‐ oxide (W–O–W) and tungsten‐ oxide‐ alumina (W–O–Al) interactions were discerned by X‐ ray photoelectron spectroscopy of the WOx/Al2O3 materials herein prepared with 1, 3, and 5 weight percentages. The 5 wt % sample displayed the highest intensity percentage of W–O–Al species correlated to the highest methanol conversion (45 %), ascribed to the appearance of the weak acidity disclosed by temperature‐ programmed desorption of ammonia.

Water adsorption lifts the (2 × 1) reconstruction of calcite(104).

The adsorption of water on calcite(104) is investigated in ultra-high vacuum by density functional theory (DFT) and non-contact atomic force microscopy (NC-AFM) in the coverage regime of up to one monolayer (ML). DFT calculations reveal a clear preference for water to adsorb on the bulk-like carbonate group rows of the (2 × 1) reconstructed surface. Additionally, an apparent water attraction due to carbonate group reorientation suggest island formation for water adsorbed on the reconstructed carbonate group rows. Experimentally, water is found to exclusively occupy specific positions within the (2 × 1) unit cell up to 0.5 ML, to form islands at coverage between 0.5 and 1 ML, and to express a (1 × 1) structure at coverage of a full monolayer.

Coverage-sensitive mechanism of electrochemical NO reduction on the SrTiO3(001) surface: a DFT investigation.

Due to its adverse environmental and human health hazards, addressing the elimination of nitric oxide (NO) has become a pressing concern for modern society. Currently, electrochemical NO reduction provides a new alternative to traditional selective catalytic reduction technology under mild reaction conditions. However, the complexity and variability of products make the coverage of NO an influencing factor that needs to be investigated. Hence, this study delves into the coverage-sensitive mechanism of electrochemical NO reduction on cost-effective perovskite catalysts, using SrTiO3 as an example, through density functional theory calculations. Phase diagrams analysis reveals that the coverage range from 0.25 to 1.00 monolayer (ML) coverage is favorable for NO adsorption. Gibbs free energy results indicate that the selectivity is significantly influenced by NO coverage. NH3 is likely to be generated at low coverage, while N2O and N2 are more likely to be produced at high coverage through a dimer mechanism. Charge analysis suggests that the charge transfer and Ti-O bond strength between reactants and catalysts are crucial factors. This work not only provides deep insights into coverage-sensitive reaction mechanisms but also is a guideline towards further rational design of high-performance perovskite catalysts.

Scalable post-treatment for improved self-assembled monolayer coverage in perovskite solar cells

The Cooled Moisture Condensation (CMC) process improves MeO–2PACz coverage on FTO by increasing surface hydroxyl groups and minimizing oxygen vacancies, resulting in more uniform self-assembled monolayer coverage and enhanced solar cell performance.

Chitosan/Alginate nanocomposites containing magnetic nanoparticles and multi-wall carbon nanotubes for efficient iron sorption

The widespread presence of iron pollution in groundwater is a major global worry, as increased levels can lead to severe health hazards. To tackle this urgent problem, we engaged in a synthesis procedure where magnetic nanoparticles (MNPs) and multi-wall carbon nanotubes (CNTs) were combined with a mixture of chitosan (CS) and sodium alginate (Alg) to produce a magnetic composite material. This novel composite, known as CS/Alg/MNPs/CNTs, was crafted with the primary aim of effectively removing iron from groundwater through adsorption. A thorough physicochemical analysis using FTIR, SEM, PSA, XRD, DSC, and TGA confirmed the composition's characteristics. The study delved into the influence of parameters such as retention time, sorbent dose, initial Fe concentration, pH, agitation rate, and temperature on the adsorption process. The Langmuir isotherm model fittingly represented the adsorption data, indicating a monolayer coverage of 12.43 mg/g and aligning with the pseudo-second-order kinetic model. Additionally, the adsorption of iron ions by the composite material was identified as a endothermic process. Furthermore, the study highlights the sorption process's inherent spontaneity and survivability, as demonstrated by the negative ΔG value. In conclusion, the results of this study indicate that the magnetic composite adsorbent (CS/Alg/MNPs/CNTs) holds considerable promise for effectively removing iron ions from groundwater, making it a viable and attractive choice for use in water purification applications.

Transition mechanism of the coverage-dependent polymorphism of self-assembled melamine nanostructures on Au(111).

Molecular self-assembled films have recently attracted increasing attention within the field of nanotechnology as they offer a route to obtain new materials. However, careful selection of the molecular precursors and substrates, as well as exhaustive control of the system evolution is required to obtain the best possible outcome. The three-fold rotational symmetry of melamine molecules and their capability to form hydrogen bonds make them suitable candidates to synthesize this type of self-assembled network. In this work, we have studied the polymorphism of melamine nanostructures on Au(111) at room temperature. We find two coverage-dependent phases: a honeycomb structure (α-phase) for submonolayer coverage and a close-packed structure (β-phase) for full monolayer coverage. A combined scanning tunnel microscopy and density functional theory based-calculations study of the transition regime where both phases coexist allows describing the mechanism underlying this coverage driven phase transition in terms of the changes in the molecular lateral tension.

Surface Functionalization of Activated Carbon Cloth for Wastewater Perchlorate Adsorption-Integrated Molecular Modeling and Experimental Energy Analysis

Perchlorate (ClO4-) in drinking water is one of the most serious issues relating to drinking water contamination. This work reports both the experimental and computational investigation of wastewater perchlorate adsorption on surfactant-modified activated carbon cloth (ACC) in order to design an efficient water treatment system. Three different types of cationic surfactants including tetramethylammonium (TMA), benzyltrimethylammonium (BTMA), and dodecyltrimethylammonium (DDTMA) were employed in this study, and the ClO4- adsorption capacity of various adsorbents was determined in batch experiments for 24 hours. Among all, DDTMA-ACC exhibits the highest ClO4- adsorption capacity (4.5 times that of pristine ACC), the highest monolayer coverage capacity ( Γ max ) value (0.59 mmol/g), an equilibrium adsorption constant ( K L ) value (35.90 L/mmol), and the lowest Δ G ads value (-26.0 KJ/mol), revealing that ClO4- has a stronger affinity for ACC functionalized by DDTMA than ACC functionalized by other surfactants. Molecular dynamics (MD) simulations were also conducted to illustrate the interactions among ClO4-, surfactants, and a model graphite surface underwater environment. The relative number density profiles and cumulative number studies demonstrate that TMA/BTMA tends to form single-layered packing near the graphite surface, while DDTMA prefers to pack in a multilayered fashion. The dense packing behavior of DDTMA on the graphite surface can enhance the ClO4- adsorption, showing good agreement with experimental results.

Interplay of Adsorption Geometry and Work Function Evolution at the TCNE/Cu(111) Interface.

The adsorption of organic electron acceptors on metal surfaces is a powerful way to change the effective work function of the substrate through the formation of charge-transfer-induced dipoles. The work function of the interfaces is hence controlled by the redistribution of charges upon adsorption of the organic layer, which depends not only on the electron affinity of the organic material but also on the adsorption geometry. As shown in this work, the latter dependence controls the work function also in the case of adsorbate layers exhibiting a mixture of various adsorption geometries. Based on a combined experimental (core-level and infrared spectroscopy) and theoretical (density functional theory) study for tetracyanoethylene (TCNE) on Cu(111), we find that TCNE adsorbs in at least three different orientations, depending on TCNE coverage. At low coverage, flat lying TCNE dominates, as it possesses the highest adsorption energy. At a higher coverage, additionally, two different standing orientations are found. This is accompanied by a large increase in the work function of almost 3 eV at full monolayer coverage. Our results suggest that the large increase in work function is mainly due to the surface dipole of the free CN groups of the standing molecules and less dependent on the charge-transfer dipole of the differently oriented and charged molecules. This, in turn, opens new opportunities to control the work function of interfaces, e.g., by synthetic modification of the adsorbates, which may allow one to alter the adsorption geometries of the molecules as well as their contributions to the interface dipoles and, hence, the work function.

Adsorption of Cd(II) ions on magnetic graphene oxide/cellulose modified with β-cyclodextrin: Analytical interpretation via statistical physics modeling and fractal like kinetic approach

In the present study, β-cyclodextrin modified magnetic graphene oxide/cellulose (CN/IGO/Cel) was fabricated for removal of Cd(II) ions. The material was characterized through various analytical techniques like FTIR, XRD, TGA/DTA, SEM, TEM, and XPS. The point of zero charge of the material was obtained as 5.38. The controllable factors were optimized by Taguchi design and optimum values were: adsorbent dose-16 mg, equilibrium time-40 min, and initial concentration of Cd(II) ions-40 mg/L. The material shows high adsorption capacity (303.98 mg/g). The good fitting of Langmuir model to adsorption data (R2 = 0.9918–0.9936) revealed the monolayer coverage on adsorbent surface. Statistical physics model M 2 showed best fitting to adsorption data (R2 > 0.997), suggesting the binding of Cd(II) ions occurred on two different receptor sites (n). Stereographically n > 1 confirming vertical multi-molecular mechanisms of Cd(II) ions adsorption on CN/IGO/Cel surface. The adsorption energies (E1 = 23.71–28.95 kJ/mol; E2 = 22.69–29.38 kJ/mol) concluded the involvement of physical forces for Cd(II) ions adsorption. Kinetic data fitted well to fractal-like pseudo first-order model (R2 > 0.9952), concluding the adsorption of Cd(II) ions occurred on energetically heterogeneous surface. The kinetic analysis shows that both the film-diffusion and pore-diffusion were responsible for Cd(II) ions uptake. XPS analysis was utilized to explain the adsorption mechanism of Cd(II) ions onto CN/IGO/Cel.

Preparation and adsorption properties of magnetic chitosan/sludge biochar composites for removal of Cu2+ ions

The magnetic chitosan/sludge biochar composite adsorbent was prepared using chitosan, Fe3O4, and sludge biochar as raw materials. The composite adsorbent was able to achieve rapid solid–liquid separation under an applied magnetic field. The morphology and microstructure of the composite adsorbent were characterized by FTIR, XRD, SEM, VSM, and BET analysis. The adsorption performance of the composite adsorbent on Cu2+ was investigated through static adsorption experiments, and the effects of adsorbent dosage, initial concentration of Cu2+, initial pH of the solution, and adsorption temperature on the adsorption efficiency of Cu2+ were discussed. The results showed that chitosan and Fe3O4 were successfully loaded on sludge biochar. When the initial concentration of Cu2+ was 30 mg/L, the dosage of the magnetic chitosan/sludge biochar composite material was 0.05 g, the adsorption time was 180 min, pH was 5, and the temperature was room temperature, the maximum removal rate of Cu2+ reached 99.77%, and the maximum adsorption capacity was 55.16 mg/g. The adsorption kinetics and adsorption isotherm data fitted well with the pseudo-second-order kinetic model and Langmuir adsorption isotherm model, indicating that the adsorption process was chemisorption with monolayer coverage.

Canola meal‐derived biochar for highly efficient dye removal and the impact of compression treatment on porous structure

AbstractBACKGROUNDThis study reported a canola meal‐based biochar adsorbent for highly efficient organic dyes removal. SEM, BET, Zeta‐potential and XPS etc. were employed to characterize the micro‐structure and chemical composition of the biochar, demonstrating the impact of compression treatment on its inner pore structure.RESULTSKOH‐activated carbonized canola meal‐compressed (KCCM‐C) showed a high specific surface area of up to 2135 m2 g−1 with an average pore size of 2.65 nm, which exhibited excellent adsorption capacity for both positively and negatively charged dyes with maximum values of 985 mg g−1 and 813 mg g−1 for methylene blue (MB) and methyl orange (MO) respectively, exceeding majority of the biochar adsorbents reported so far. The dye adsorption behavior was dominated by monolayer coverage with both physical adsorption and chemisorption. Langmuir isotherm model fitted well with the adsorption data, and the adsorption kinetic followed the pseudo‐second‐order model. Negative Gibbs free energy and positive enthalpy change indicated that the adsorption was a spontaneous endothermic process. XPS analysis indicated that the amino groups of proteins inside the canola meal as the active sites were oxidated by KOH prior to other components, while the compact structure of compressed canola meal was more conducive to the formation of uniform meso‐pores inside canola meal‐derived biochar improving its adsorption performance.CONCLUSIONSThis high‐quality porous biochar, directly synthesized from the compressed canola meal, simplifies the grinding process and presents a valuable opportunity for the value‐added application of canola meal. © 2023 Society of Chemical Industry (SCI).

Activation of Glassy Carbon Surfaces by Alkaline Anodization Enhances Dopamine Adsorption and Electron‐Transfer Kinetics

AbstractSurface activation can increase the sensitivity of carbon electrodes for electrochemical detection of neurotransmitters, such as dopamine (DA). However, fully understanding the changes in DA behavior after electrode activation remains elusive. Herein, we use scanning electrochemical cell microscopy (SECCM) to investigate DA adsorption and electrochemistry on glassy carbon (GC) after anodization in alkaline media. Through localized microscale anodization to different extents (two distinct times and same potential), followed by spatially‐resolved SECCM, we achieve direct visualization of DA electrochemistry across unmodified and anodized GC regions, with complementary microscopy and spectroscopy unraveling the effects of co‐located carbon surface chemistry and structure. Our findings reveal that short anodization times (60 s) enhance DA adsorption, rapidly reaching nearly complete monolayer coverage. Longer anodization (300 s) induces further changes in electrochemical surface area so that although the DA surface concentration increased based on geometric area, it was smaller based on specific surface area. This work highlights that even small differences in surface properties after activation can significantly impact electrode performance for neurotransmitter sensing, and separating the individual contribution of chemical and topographical factors remains challenging. SECCM provides an excellent platform to assess activation methods in a high throughput format to help accelerate the discovery of optimal modified electrodes.