Showing papers on "Chemisorption published in 2010"

First-principles study of metal–graphene interfaces

Abstract: Metal-graphene contact is a key interface in graphene-based device applications, and it is known that two types of interfaces are formed between metal and graphene. In this paper, we apply first-principles calculations to twelve metal-graphene interfaces and investigate the detailed interface atomic and electronic structures of physisorption and chemisorption interfaces. For physisorption interfaces (Ag, Al, Cu, Cd, Ir, Pt, and Au), Fermi level pinning and Pauli-exclusion-induced energy-level shifts are shown to be two primary factors determining graphene’s doping types and densities. For chemisorption interfaces (Ni, Co, Ru, Pd, and Ti), the combination of Pauli-exclusion-induced energy-level shifts and hybridized states’ repulsive interactions lead to a band gap opening with metallic gap states. For practical applications, we show that external electric field can be used to modulate graphene’s energy-levels and the corresponding control of doping or energy range of hybridization.

Adsorption of molecular oxygen on doped graphene: atomic, electronic and magnetic properties

Abstract: Adsorption of molecular oxygen on B-, N-, Al-, Si-, P-, Cr- and Mn-doped graphene is theoretically studied using density-functional theory in order to clarify if ${\text{O}}_{2}$ can change the possibility of using doped graphene for gas sensors, electronic, and spintronic devices. ${\text{O}}_{2}$ is physisorbed on B-, and N-doped graphene with small adsorption energy and long distance from the graphene plane, indicating the oxidation will not happen; chemisorption is observed on Al-, Si-, P-, Cr- and Mn-doped graphene. The local curvature caused by the large bond length of X-C (X represents the dopants) relative to C-C bond plays a very important role in this chemisorption. The chemisorption of ${\text{O}}_{2}$ induces dramatic changes of electronic structures and localized spin polarization of doped graphene, and in particular, chemisorption of ${\text{O}}_{2}$ on Cr-doped graphene is antiferromagnetic. The analysis of electronic density of states shows the contribution of the hybridization between O and dopants is mainly from the $p$ or $d$ orbitals. Furthermore, spin density shows that the magnetization locates mainly around the doped atoms, which may be responsible for the Kondo effect. These special properties supply a good choice to control the electronic properties and spin polarization in the field of graphene engineering.

Communications: Exceptions to the d-band model of chemisorption on metal surfaces: The dominant role of repulsion between adsorbate states and metal d-states

Abstract: We show that there is a family of adsorbate-substrate systems that do not follow the trends in adsorption energies predicted by the d-band model. A physically transparent model is used to analyze this phenomenon. We found that these adsorbate-substrate pairs are characterized by the repulsive interaction of the substrate d-band with the renormalized adsorbate states. The exceptions to the d-band model are mainly associated with the adsorbates having almost completely filled valence shell, and the substrates with nearly fully occupied d-band, e.g., OH, F, or Cl adsorption on metals and alloys characterized by d(9) or d(10) substrate surface atoms.

Thermodynamic and kinetic studies of cadmium adsorption from aqueous solution onto rice husk

Abstract: The adsorption behavior of rice husk for cadmium ions from aqueous solutions has been investigated as a function of appropriate equilibrium time, adsorbent dose, temperature, adsorbate concentrations and pH in a batch system. Studies showed that the pH of aqueous solutions affected cadmium removal with the result that removal efficiency increased with increasing solution pH. The maximum adsorption was 98.65% at solution pH 6, contact time 60 min and initial concentration of 25 mg/L. The experimental data were analysed by the Langmuir, Freundlich and Temkin models of adsorption. The characteristic parameters for each isotherm and related correlation coefficients have been determined. Thermodynamic parameters such as, and have also been evaluated and it has been found that the sorption process was feasible, spontaneous and exothermic in nature. The kinetics of the sorption were analysed using the pseudo-first order and pseudo-second order kinetic models. Kinetic parameters, rate constants, equilibrium sorption capacities and related correlation coefficients for each kinetic model were calculated and discussed. It was shown that the adsorption of cadmium could be described by the pseudo-second order equation, suggesting that the adsorption process is presumably a chemisorption. The rice husk investigated in this study showed good potential for the removal of cadmium from aqueous solutions. The goal for this work is to develop inexpensive, highly available, effective metal ion adsorbents from natural waste as alternative to existing commercial adsorbents.

Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH3 Synthesis with Ru Nanoparticles

Abstract: Carbon nanotubes (CNTs) have been shown to modify some properties of nanomaterials and to modify chemical reactions confined inside their channels, which are formed by curved graphene layers. Here we studied ammonia synthesis over Ru as a probe reaction to understand the effect of the electron structure of CNTs on the confined metal particles and their catalytic activity. The catalyst with Ru nanoparticles dispersed almost exclusively on the exterior nanotube surface exhibits a higher activity than the CNT-confined Ru, although both have a similar metal particle size. Characterization with TEM, N-2 physisorption, H-2 chemisorption, temperature-programmed reduction. CO adsorption microcalorimetry, and first-principles calculations suggests that the outside Ru exhibits a higher electron density than the inside Ru. As a result, the dissociative adsorption of N-2 which is an electrophilic process and the rate-determining step of ammonia synthesis. is more facile over the outside Ru than that over the inside one.

Removal of Mn 2+ from aqueous solution by manganese oxide coated zeolite

Abstract: The preparation, characterization, and adsorption properties of Mn 2+ by manganese oxide coated zeolite (MOCZ) and its ability in removing Mn 2+ by adsorption were investigated. Characterization analyses were used to monitor the surface properties (and their changes) of the coated layer and metal adsorption sites on the surface of MOCZ. The adsorption experiments were carried out as a function of solution pH, adsorbent concentration and contact time. Binding of Mn 2+ ions onto MOCZ was highly pH dependent with an increase in the extent of adsorption with the pH of the media investigated. After the Mn 2+ adsorption by MOCZ, the medium pH decreased and enhanced with increasing adsorbent concentration. The pseudosecond-order model fitted better among all the kinetic models suggesting that the adsorption mechanism might be a chemisorption process. The equilibrium data showed excellent correlation for both Langmuir and Freundlich isotherm model and this implies both monolayer adsorption and a heterogeneous surface existence in MOCZ. At pH = 6, the Mn

Metal Clusters in Catalysis III. — Clusters as Models for Chemisorption Processes and Heterogeneous Catalysis

Abstract: An analysis of the intrinsic structural, stereochemical and thermodynamic features of metal surfaces vis a vis those of discrete metal clusters suggests that the clusters are attractive models of metal surfaces for chemisorption and heterogeneous catalysis. Chemical and catalytic properties of the two classes are compared.

Pathways for Oxygen Incorporation in Mixed Conducting Perovskites: A DFT-Based Mechanistic Analysis for (La, Sr)MnO3−δ

Abstract: An extensive set of DFT calculations on LaMnO3 slabs has been generated and used as a basis to identify the most probable reaction mechanism for oxygen incorporation into (La, Sr)MnO3−δ cathode materials. MnO2[001] is found to be the most stable surface termination under fuel cell operation conditions (high temperature, high pO2, cubic unit cell). Chemisorption leading to the formation of O2−, O22−, and O− atop Mn is exothermic, but due to the negative adsorption entropy and electrostatic repulsion the levels of coverage of molecular oxygen adsorbates are low (in the few percent range). Under typical solid oxide fuel cell conditions, a mechanism in which the encounter of O− with a surface oxygen vacancy at the surface is rate-determining exhibits the fastest rate. The variation of the reaction rate and preferred mechanism(s) with adsorbate and point defect concentrations is discussed.

Counterintuitive sensing mechanism of ZnO nanoparticle based gas sensors

Abstract: Zinc oxide nanoparticles are prepated by calcining zinc hydrocarbonate precursors at 300-700 degrees C (ZnO300-700). and corresponding gas sensing property are tested at 300 degrees C by using formaldehyde as the probe Although the nanoparticle sizes are found to gradually increase with calcination temperature, the sensor measurements reveal the size-independent behavior that ZnO500 and ZnO300 have the highest and lowest responses, respectively Spectroscopic characterization further reveals nonstoichiometric compositions of ZnO nanoparticles ZnO300 has the largest excess oxygen (oxygen interstitial, O(1)). whereas ZnO500 has the largest excess zinc (oxygen vacancy, V(o) and/or zinc interstitial, Zn(1)) Accordingly, a new sensing mechanism is proposed for ZnO nanoparticle sensois Excess zinc favors chemisorption of oxygen onto the nanoparticle surface, leading to reacting with mot e formaldehyde molecules to get a high signal On the contrary, excess oxygen inhibits free oxygen to be chemisorbed onto the nanoparticle surface, and thus decreases the gas response Finally, this new sensing mechanism is verified by testing gas response of ZnO500 nanoparticles annealed at different atmospheres (C) 2010 Elsevier B V All lights reserved

Bimetallic Ru Electrocatalysts for the OER and Electrolytic Water Splitting in Acidic Media

Abstract: We have explored bimetallic Ru-M oxygen evolution reaction (OER) electrocatalysts for use in water splitting in acidic electrolytes. Using an electrochemical multielectrode cell, we investigated the OER activity of selected compositions of seven binary alloy systems, Ru-M (M = Pd, Ir, Cu, Co, Re, Cr, Ni). Benchmarked using pure Ru electrocatalysts, Ru-Co, Ru-Ir, and Ru-Cu exhibited improved Ru mass-based catalytic activities. Structural studies of the precursor alloys indicated the presence of hexagonal and cubic mixed metal phases. We hypothesize that the secondary metal component modulates the chemisorption energy of oxygen, which was suggested to be a sensitive rate controlling parameter in the OER catalysis and favors the formation of atomic oxygen O ad and possibly HOO ad species rather than OH ad species on the oxide catalyst.

Application of the ReaxFF Reactive Force Field to Reactive Dynamics of Hydrocarbon Chemisorption and Decomposition

Abstract: We report here reactive dynamics (RD) simulations of the adsorption and decomposition of a gas of 20−120 methane, ethyne, ethene, benzene, cyclohexane, or propene molecules interacting with a 21 A diameter nickel nanoparticle (468 atoms). These RD simulations use the recently developed ReaxFF reactive force field to describe decomposition, reactivity, and desorption of hydrocarbons as they interact with nickel surfaces. We carried out 100 ps of RD as the temperature is ramped at a constant rate from 500 to 2500 K (temperature programmed reactions). We find that all four unsaturated hydrocarbon species chemisorb to the catalyst particle with essentially no activation energy (attaching to the surface through π electrons) and then proceed to decompose by breaking C−H bonds to form partially dehydrogenated species prior to decomposition to lower order hydrocarbons. The eventual breaking of C−C bonds usually involves a surface Ni atom inserting into the C−C bond to produce an atomic C that simultaneously with ...

Thermokinetic analysis of the CO2 chemisorption on Li4SiO4 by using different gas flow rates and particle sizes.

Abstract: Lithium orthosilicate (Li(4)SiO(4)) was synthesized by solid-state reaction and then its CO(2) chemisorption capacity was evaluated as a function of the CO(2) flow rate and particle size. Initially, a Li(4)SiO(4) sample, with a total surface area of 0.4 m(2)/g, was used to analyze the CO(2) chemisorption, varying the CO(2) flow between 30 and 200 mL/min. Results showed that CO(2) flows modify the kinetic regime from which CO(2) capture is controlled. In the first moments and at low CO(2) flows, the CO(2) capture is controlled by the CO(2) diffusion through the gas-film system, whereas at high CO(2) flows it is controlled by the CO(2) chemisorption reaction rate. Later, at larger times, once the carbonate-oxide external shell has been produced the whole process depends on the CO(2) chemisorption kinetically controlled by the lithium diffusion process, independently of the CO(2) flow. Additionally, thermokinetic analyses suggest that temperature induces a CO(2) particle surface saturation, due to an increment of CO(2) diffusion through the gas-film interface. To elucidate this hypothesis, the Li(4)SiO(4) sample was pulverized to increase the surface area (1.5 m(2)/g). Results showed that increasing the surface particle area, the saturation was not reached. Finally, the enthalpy activation (DeltaH(double dagger)) values were estimated for the two CO(2) chemisorption processes, the CO(2) direct chemisorption produced at the Li(4)SiO(4) surface, and the CO(2) chemisorption kinetically controlled by the lithium diffusion, once the carbonate-oxide shell has been produced.

Density functional theory study of ATA, BTAH, and BTAOH as copper corrosion inhibitors: adsorption onto Cu(111) from gas phase.

Abstract: A low-coverage gas-phase adsorption of three corrosion inhibitors-3-amino-1,2,4-triazole (ATA), benzotriazole (BTAH), and 1-hydroxybenzotriazole (BTAOH)-on perfect Cu(111) surface has been studied and characterized using density functional theory calculations. We find that the molecules in neutral form chemisorb weakly to the perfect surface in an upright geometry. The strength of the chemisorption increases in the order BTAH < BTAOH < ATA with adsorption energies of -0.40, -0.53, and -0.60 eV, respectively. The molecules bond to the surface with triazole nitrogen atoms and also through X-H···Metal hydrogen bonds (X = N or O). In addition to chemisorption, BTAH and BTAOH can also physisorb with the molecular plane being nearly parallel to the surface and the energies of the physisorption are -0.72 and -0.97 eV, respectively, hence being more exothermic than the corresponding chemisorption energies. On the other hand, the molecules in dehydrogenated form chemisorb strongly to the surface and the strength of the chemisorption increases in the order BTAO· < ATA· < BTA· with the adsorption energies of -1.65, -2.22, and -2.78 eV, respectively. This order is compatible with the trend of experimentally observed corrosion inhibition effectiveness on copper in near-neutral chloride solutions. Although the calculations are performed at the metal/vacuum interface, they provide enough insight to rationalize why in some experiments the BTAH was observed to be adsorbed with an upright geometry and in the others with parallel geometry.

Reactivities of Sites on (5,5) Single-Walled Carbon Nanotubes with and without a Stone-Wales Defect

Abstract: The reactivities of various carbon sites on (5,5) single-walled carbon nanotubes (SWCNT) of C70H20 with and without a Stone-Wales defect have been predicted computationally. The properties determined include the average local ionization energy Is(r) and pyramidalization angle θP on the surfaces of the bare tubes, the chemisorption energies, bond lengths, stretching frequencies for chemisorbed H and F atoms, and the effects of H and F chemisorption upon the HOMO−LUMO energy gaps. There is a good correlation between the minima of the local ionization energy and the chemisorption energies at different carbon sites, indicating that Is(r) provides an effective means for rapidly and inexpensively assessing the relative reactivities of the carbon sites of SWCNTs. The pyramidalization angle (θP), which is a measure of local curvature, also shows a relationship to site reactivity. The most reactive carbon site, identified by having the lowest Is(r) and largest θP, is in the Stone-Wales defect region, which also...

Competing strain effects in reactivity of LaCoO 3 with oxygen

Abstract: Planar strain effects on oxygen-vacancy formation and oxygen adsorption on ${\text{LaCoO}}_{3}$ are shown to manifest through competing mechanisms. Through first-principles calculations, we demonstrate that these unit processes are facilitated by elastic stretching. On the other hand, spin-state transitions and Co-O bond exchange hinder these processes by trapping the lattice oxygen with increasing tensile strain. A transition from chemisorption to physisorption of the oxygen molecule is identified at high strains. Insights on charge-density profiles, density of electronic states, and stress thresholds suggest the possibility of tuning strain-mediated reactivity in ${\text{LaCoO}}_{3}$ and related perovskite oxides.