Adsorption Site Changes Research Articles

Risk assessment for the long-term stability of fly ash-based cementitious material containing arsenic: Dynamic and semidynamic leaching

Fly ash from municipal solid waste incineration (MSWIFA) contains leachable heavy metals (HMs), and the environmental risk of contained HMs is an important concern for its safe treatment and disposal. This paper presents a dynamic leaching test of fly ash-based cementitious materials containing arsenic (FCAC) in three particle sizes based on an innovative simulation of two acid rainfall conditions to investigate the long-term stability of FCAC under acid rain conditions. As well as semi-dynamic leaching test by simulating FCAC in three scenarios. Furthermore, the long-term stability risk of FCAC is evaluated using a sequential extraction procedure (SEP) and the potential risk assessment index. Results showed that the Al3+ in the FCAC dissolved and reacted with the OH− in solution to form Al(OH)3 colloids as the leaching time increased. Moreover, the oxidation of sulfide minerals in the slag produced oxidants, such as H2SO4 and Fe2(SO4)3, which further aggravated the oxidative dissolution of sulfides, thereby resulting in an overall decreasing pH value of the leachate. In addition, due to the varying particle sizes of the FCAC, surface area size, and adsorption site changes, the arsenic leaching process showed three stages of leaching characteristics, namely, initial, rapid, and slow release, with a maximum leaching concentration of 2.42 mg/L, the cumulative release of 133.78 mg/kg, and the cumulative release rate of 2.32%. The SEP test revealed that the reduced state of HMs in the raw slag was lowered substantially, and the acid extractable state and residual state of HMs were increased, which was conducive to lessening the risk of FCAC. Overall, the geological polymerization reaction of MSWIFA is a viable and promising solution to stabilize mining and industrial wastes and repurpose the wastes into construction materials.

Interfacial quantum chemical characterization of aromatic organic matter adsorption on oxidized microplastic surfaces

Examining the adsorption efficiency of individual contaminants on microplastics (MPs) is resource-intensive and time-consuming. To address this challenge, combined laboratory adsorption experiments with model simulations were performed to investigate the adsorption capacities and mechanisms of MPs before and after aging. Our adsorption experiments revealed that aged polyethylene (PE) and polyvinyl chloride (PVC) MPs exhibited increased adsorption capacity for benzene, phenol, and naphthalene. Additionally, density functional theory (DFT) simulations provided insights into changes in adsorption sites, adsorption energy, and charge density on MPs. The π bond of the benzene ring emerged as a pivotal factor in the adsorption process, with van der Waals forces exerting dominant influence. For instance, the adsorption energy of benzene on pristine PE was −0.01879 eV. When oxidized groups, such as hydroxyl, carbonyl, and carboxyl, on the surface of aged PE became the adsorption sites, the adsorption energy increased to −0.06976, −0.04781, and −0.04903 eV, respectively. Regions with unoxidized functional groups also exhibited higher adsorption energies than pristine PE. These results indicated that aged PE had a stronger affinity for benzene compared to pristine PE, enhancing its adsorption. Charge density difference and energy density of states corroborated this observation, revealing larger π-bond charge accumulation areas for benzene on aged PE, suggesting stronger dipole interactions and enhanced adsorption. Similar trends were observed for phenol and naphthalene. In summary, the DFT calculations aligned with the adsorption experiment findings, confirming the effectiveness of simulation methods in predicting changes in the adsorption performance of aged MPs.

Adsorption configurations of Co-phthalocyanine on In2O3(111)

Indium oxide offers optical transparency paired with electrical conductivity, a combination required in many optoelectronic applications. The most-stable In2O3(111) surface has a large unit cell (1.43 nm lattice constant). It contains a mixture of both bulk-like and undercoordinated O and In atoms and provides an ideal playground to explore the interaction of surfaces with organic molecules of similar size as the unit cell. Non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), and density functional theory (DFT) were used to study the adsorption of Co-phthalocyanine (CoPc) on In2O3(111). Isolated CoPc molecules adsorb at two adsorption sites in a ∼7:3 ratio. The Co atom sits either on top of a surface oxygen (‘F configuration’) or indium atom (‘S configuration’). This subtle change in adsorption site induces different bending of the molecules and differences in their electronic structure. According to DFT, the lowest unoccupied molecular orbital of the undistorted gas-phase CoPc remains mostly unaffected in the F configuration but is filled by one electron in S configuration. At coverages up to one CoPc molecule per substrate unit cell, a mixture of domains with molecules in F and S configuration are found. Molecules in F configuration first condense into a poorly ordered F-(2 × 2) structure and finally rearrange into an F-(1 × 1) structure in which the molecules partially overlap. The only ordered structure of molecules adsorbed in S configuration is an S-(1 × 1) superstructure at full monolayer coverage.

Functionalized construction of biochar with hierarchical pore structures and surface O-/N-containing groups for phenol adsorption

Volatile organic compounds (VOCs) seriously endanger human health due to their bioaccumulation and carcinogenic effects, while biochar adsorption method is regarded as one of the most potential VOCs removal technologies. In order to clarify the structure-activity relationship of biochar adsorption of VOCs, corn straw was used as a biomass raw material for chemically activated biochar preparation and liquid-phase adsorption experiments. The simulations including molecular dynamics simulations and quantum chemical simulations were carried out to verify the relevant conclusions. The results show that abundant O-/N-containing functional groups are constructed on the pore surface of chemically activated biochar, forming a typical hierarchical-functionalized pore structure. The hierarchical-functionalized pore structure has an ideal promotion effect on the adsorption of phenol. Specifically, mesoporous structure on biochar surface mainly determines adsorption of phenol. The hydroxyl group strengthens the adsorption of phenol, due to the increased electrostatic effect, while N-6 enhances the van der Waals force for the change of adsorption site, thereby strengthening the adsorption of phenol. This work provides theoretical support for the practical application of biochar for efficient VOCs removal technology.

Hiroshi Kitagawa.

"The secrets of being a successful scientist are dreams, imagination, and lots of effort. The biggest challenge facing scientists is the room-temperature superconductor …" Find out more about Hiroshi Kitagawa in his Author Profile.

Frontispiece: A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity

Frontispiece: A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity

Frontispiz: A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity

Frontispiz: A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity

A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity.

Ru is an important catalyst in many types of reactions. Specifically, Ru is well known as the best monometallic catalyst for oxidation of carbon monoxide (CO) and has been practically used in residential fuel cell systems. However, Ru is a minor metal, and the supply risk often causes violent fluctuations in the price of Ru. Performance-improved and cost-reduced solid-solution alloy nanoparticles of the Cu-Ru system for CO oxidation are now presented. Over the whole composition range, all of the Cux Ru1-x nanoparticles exhibit significantly enhanced CO oxidation activities, even at 70 at % of inexpensive Cu, compared to Ru nanoparticles. Only 5 at % replacement of Ru with Cu provided much better CO oxidation activity, and the maximum activity was achieved by 20 at % replacement of Ru by Cu. The origin of the high catalytic performance was found as CO site change by Cu substitution, which was investigated using in situ Fourier transform infrared spectra and theoretical calculations.

A CO Adsorption Site Change Induced by Copper Substitution in a Ruthenium Catalyst for Enhanced CO Oxidation Activity

AbstractRu is an important catalyst in many types of reactions. Specifically, Ru is well known as the best monometallic catalyst for oxidation of carbon monoxide (CO) and has been practically used in residential fuel cell systems. However, Ru is a minor metal, and the supply risk often causes violent fluctuations in the price of Ru. Performance‐improved and cost‐reduced solid‐solution alloy nanoparticles of the Cu‐Ru system for CO oxidation are now presented. Over the whole composition range, all of the CuxRu1−x nanoparticles exhibit significantly enhanced CO oxidation activities, even at 70 at % of inexpensive Cu, compared to Ru nanoparticles. Only 5 at % replacement of Ru with Cu provided much better CO oxidation activity, and the maximum activity was achieved by 20 at % replacement of Ru by Cu. The origin of the high catalytic performance was found as CO site change by Cu substitution, which was investigated using in situ Fourier transform infrared spectra and theoretical calculations.

Behavior of aluminum adsorption and incorporation at GaN(0001) surface: First-principles study

First-principles calculations are performed to study the energetics and atomic structures of aluminum adsorption and incorporation at clean and Ga-bilayer GaN(0001) surfaces. We find the favorable adsorption site changes from T4 to T1 as Al coverage increased to 1 monolayer on the clean GaN(0001) surface, and a two-dimensional hexagonal structure of Al overlayer appears. It is interesting the Al atoms both prefer to concentrate in one deeper Ga layer of clean and Ga-bilayer GaN(0001) surface, respectively, while different structures could be achieved in above surfaces. For the case of clean GaN(0001) surface, corresponding to N-rich and moderately Ga-rich conditions, a highly regular superlattice structure composed of wurtzite GaN and AlN becomes favorable. For the case of Ga-bilayer GaN(0001) surface, corresponding to extremely Ga-rich conditions, the Ga bilayer is found to be sustained stable in Al incorporating process, leading to an incommensurate structure directly. Furthermore, our calculations provide an explanation for the spontaneous formation of ordered structure and incommensurate structure observed in growing AlGaN films. The calculated results are attractive for further development of growth techniques and excellent AlGaN/GaN heterostructure electronic devices.

Selenium adsorption on Mo(110): A first-principles investigation

Selenium adsorption on molybdenum surfaces is a relevant process in the production of thin-film solar cells, in particular as far as the formation of the layered compound MoSe${}_{2}$ is concerned. In this paper we investigate the energetics of Se adsorption on the (110) surface of molybdenum using first-principles calculations in the two limiting cases of low and high coverage, and we establish a comparison with the more extensively investigated case of sulfur adsorption at submonolayer coverage. The studied system provides the opportunity for testing the most crucial approximations, namely, the choice of the exchange-correlation functional and the pseudopotential generation. We find that semicore states of molybdenum have an influence on calculated surface energies and, to a lesser extent, on adsorption energies. We compare some more or less popular semilocal exchange-correlation functionals, including one recently proposed as an improvement for quasi-two-dimensional systems. The results show that the preferred adsorption site changes with coverage and suggest a strong variation of the adsorption energy with coverage.

Low temperature motion of hydrogen on metal surfaces signals breakdown of quantum mechanics in 3+1 dimensions

The low temperature motion of hydrogen on solid metal surfaces displays some unexplained experimental features: in the quantum diffusion regime more than nine orders of magnitude difference between the diffusion rates on different metal surfaces have been measured, the lowest diffusion rates being established in the low temperature scanning tunnelling microscope. Furthermore telegraph-signal-like adsorption site change, rather than Rabi oscillations predicted by Schrödinger equation in 3+1 dimensions, is observed, signaling the breakdown of quantum mechanics in 3+1 dimensions. A theory is presented to resolve these problems, involving the entanglement of the adsorbate motion to gravitons in high-dimensional spacetime. Soft local massive gravonons, induced in the presence of the adsorbate, determine the time scale for surface diffusion. The γη-model is used for the evaluation of the soft gravonon modes. Weak and local entanglement of the adsorbate motion with a nearly degenerate graviton continuum of high density of states are the conditions for the telegraph-signal-like time development of adsorption site change. In contrast to the Copenhagen interpretation of quantum mechanics, this apparent "classical" behaviour of the adsorbate in 3+1 dimensional spacetime is the result of the solution of Schrödinger's time dependent equation in high-dimensional spacetime.

Molecular Design Rule of Phthalocyanine Dyes for Highly Efficient Near‐IR Performance in Dye‐Sensitized Solar Cells

A series of zinc-phthalocyanine sensitizers (PcS16-18) with different adsorption sites have been designed and synthesized in order to investigate the dependence of adsorption-site structures on the solar-cell performances in zinc-phthalocyanine based dye-sensitized solar cells. The change of adsorption site affected the electron injection efficiency from the photoexcited dye into the nanocrystalline TiO2 semiconductor, as monitored by picosecond time-resolved fluorescence spectroscopy. The zinc-phthalocyanine sensitizer PcS18, possessing one carboxylic acid directly attached to the ZnPc ring and six 2,6-diisopropylphenoxy units, showed a record power conversion efficiency value of 5.9 % when used as a light-harvesting dye on a TiO2 electrode under one simulated solar condition.

Vibrational Investigation of Catalyst Surfaces: Change of the Adsorption Site of CO Molecules upon Coadsorption

The understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase is one of the foremost goals of surface science. Adsorption on solid surfaces is the first step in catalytic reactions. Therefore, the individuation of adsorption sites for reactive chemical species is essential information to tailor catalytic properties of surfaces and interfaces. As a matter of fact, the change in adsorption site often implies a different reactivity for chemisorbed adsorbates and a selective catalytic activity. In this Feature Article, we evidence how vibrational spectroscopy can be used for individuating adsorption sites in coadsorption systems on catalyst surfaces. In particular, we studied CO coadsorption with oxygen, nitrogen, and hydrogen on the Ni(111) and the Ni(100) surfaces. Our attention was focused on the determination of CO adsorption sites in the various investigated systems. For CO adsorbed alone on the substrate, the preferential adsorption sites are the 3-fold hollow on the (111) face and the atop and bridge for the (100) surface. Striking changes in the CO adsorption site occur whenever CO is coadsorbed with other chemical species. In the CO + O coadsorption system, atop sites are populated by CO on the Ni(111) surface, while bridge and 4-fold hollow sites are occupied on Ni(100). In the CO + N phase on Ni(111), CO molecules occupy the bridge site of the pseudo-(100) reconstructed surface. For a H-precovered Ni(100) surface at 150 K, the C–O stretching frequency is near its gas-phase value, thus suggesting the occurrence of a weakly bonded CO phase without changes of adsorption sites.

The interaction of CO with PdAg/Pd(111) surface alloys—A case study of ensemble effects on a bimetallic surface

The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.

First-principles calculation and transmission electron microscopy observation for hydrogen adsorption on carbon nanowalls

Transmission electron microscopy (TEM) technique was used to investigate the structure of carbon nanowalls (CNWs). The TEM observation clearly indicated the existence of the bended graphene structure in boundary regions between the crystallites of CNWs. According to this TEM result, the first-principles calculation was employed for the bended coronene molecule as the model of the boundary region between the crystallites of CNWs, to elucidate the mechanism of the hydrogen adsorption to CNWs. The hydrogen adsorption energies onto both on-top and hollow sites become greater as the bending angle increases, because the electronic structure of the carbon atom at the adsorption site changes from sp2 to sp3 hybridization character by natural bond orbital analysis. Our computational result is reasonably consistent with the specific feature of hydrogen adsorption to CNWs, which had been reported in our previous work by ultraviolet photoelectron spectroscopy and temperature program desorption measurements by Kinoshita et al.[Chem. Phys. Lett. 450, 360 (2008)].

Direct observation of conductance fluctuations of a single-atom tunneling contact

Using a cryogenic scanning tunneling microscope, the dynamics of lateral translations of a single silver atom adsorbed on Ag(111) were investigated with a time resolution of 10 ns. Elevated tunneling currents passed through the tip close to the adsorbed atom induce sudden adsorption site changes, which are reflected by two-level fluctuations of the tunneling conductance. The voltage dependence of the fluctuation rate reveals different local heating of the adsorbed atom in different positions relative to the tip.

Site selectivity of benzene adsorption on Ni(111) studied by high-resolution x-ray photoelectron spectroscopy

The adsorption and thermal evolution of deuterated and nondeuterated benzene on Ni(111) has been studied by in situ high-resolution x-ray photoelectron spectroscopy. The recorded C $1s$ spectra show vibrational fine structure due to vibrational excitations of the C-H bonds in the ionic final state. Furthermore, different adsorption sites can be identified, since carbon atoms within the same molecule, but with different positions relative to the substrate have different C $1s$ binding energies. At $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for both ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ and ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$ a well ordered $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7)}$ $R19.1\ifmmode^\circ\else\textdegree\fi{}$ superstructure is observed at saturation of the chemisorbed layer, with a nominal coverage of 0.143 ML. From a quantitative analysis of the XP spectra collected during adsorption one finds that up to a coverage of 0.10 ML benzene occupies only bridge adsorption sites. At higher coverage, a change to hcp hollow sites, accompanied by a rotation of the molecules by 30\ifmmode^\circ\else\textdegree\fi{}, occurs for ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ and ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$. In contrast to these results, an isotopic effect is found for adsorption at $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. While the adsorption behavior for ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$ is similar as at $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ no change in adsorption site is observed, i.e., up to saturation of the chemisorbed layer bridge sites are occupied. Only upon heating, a site change from bridge to hcp hollow of the adsorbed ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ is induced, starting around $180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A simple explanation for the observed behavior is proposed.

Hydrogen adsorption and diffusion on Pt {111} and PtSn {111}

PtSn {111} forms two ordered surface alloys namely p(2 × 2) and (√3 × √3R30°) and such bimetallic catalysts are often used in industrially catalysed reactions, such as hydrogenations, to improve performance although the reasons behind the improvements are generally not well understood. In this study density functional theory calculations have been performed for hydrogen adsorption at various sites on Pt {111} and the ordered PtSn {111} surfaces in order to characterise the differences between the pure and doped surfaces. The structural, bonding, electronic and vibrational characteristics of each system are presented indicating that there are no significant bonding differences when adsorption occurs at platinum sites on the surface. Where adsorption occurs at a tin site on the doped surface the adsorption energy is negative or the adsorption site changes to one involving only platinum atoms. Nudged elastic band calculations were used to determine the barrier to diffusion of the hydrogen atoms on the surface. The diffusion calculations show significant differences in the mobility of hydrogen providing a possible explanation for the changes in catalytic action on tin doping of platinum.

Adsorbate-induced roughening of nanosized palladium particles after coherent laser excitation

Femtosecond laser induced adsorption site changes of CO at nanoparticulate palladium aggregates have been investigated using laser light at λ=400 nm and pulse lengths of 70 fs. The average sizes of the aggregates grown by evaporating palladium atoms onto an epitaxial Al2O3 film on NiAl(110) were varied between 100 and 6000 atoms per island. Amorphous aggregates grown at 120 K and crystalline aggregates grown at 300 K have been investigated. Beyond laser-induced migration of CO already reported for nanosecond experiments, adsorbate-induced surface roughening is apparent from Fourier-transform infrared reflection absorption spectroscopy. The laser-induced processes are correlated to the presence of higher local adsorbate densities and a substantial population of edge sites. A model is suggested attributing surface roughening to the formation of local hot spots after coherent electronic excitation via heating through highly vibrationally excited adsorbate states and energetic channelling along chains of elevated local adsorbate densities. The processes are most efficient for intermediate aggregate sizes. In contrast to nanosecond experiments, a few percent of CO molecules are desorbed during the femtosecond experiments from smaller aggregates.