Showing papers on "Chemisorption published in 2013"

Adsorption of divalent metal ions from aqueous solutions using graphene oxide

Abstract: The adsorptive properties of graphene oxide (GO) towards divalent metal ions (copper, zinc, cadmium and lead) were investigated GO prepared through the oxidation of graphite using potassium dichromate was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR) The results of batch experiments and measurements by flame atomic absorption spectrometry (F-AAS) indicate that maximum adsorption can be achieved in broad pH ranges: 3–7 for Cu(II), 5–8 for Zn(II), 4–8 for Cd(II), 3–7 for Pb(II) The maximum adsorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) on GO at pH = 5 are 294, 345, 530, 1119 mg g−1, respectively The competitive adsorption experiments showed the affinity in the order of Pb(II) > Cu(II) ≫ Cd(II) > Zn(II) Adsorption isotherms and kinetic studies suggest that sorption of metal ions on GO nanosheets is monolayer coverage and adsorption is controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO Chemisorption was confirmed by XPS (binding energy and shape of O1s and C1s peaks) of GO with adsorbed metal ions The adsorption experiments show that the dispersibility of GO in water changes remarkably after complexation of metal ions After adsorption, the tendency to agglomerate and precipitate is observed Excellent dispersibility of GO and strong tendency of GO–Me(II) to precipitate open the path to removal of heavy metals from water solution Potential application of GO in analytical chemistry as a solid sorbent for preconcentration of trace elements and in heavy metal ion pollution cleanup results from its maximum adsorption capacities that are much higher than those of any of the currently reported sorbents

Surface Facet of Palladium Nanocrystals: A Key Parameter to the Activation of Molecular Oxygen for Organic Catalysis and Cancer Treatment

Abstract: In many organic reactions, the O2 activation process involves a key step where inert ground triplet O2 is excited to produce highly reactive singlet O2. It remains elusive what factor induces the change in the electron spin state of O2 molecules, although it has been discovered that the presence of noble metal nanoparticles can promote the generation of singlet O2. In this work, we first demonstrate that surface facet is a key parameter to modulate the O2 activation process on metal nanocrystals, by employing single-facet Pd nanocrystals as a model system. The experimental measurements clearly show that singlet O2 is preferentially formed on {100} facets. The simulations further elucidate that the chemisorption of O2 to the {100} facets can induce a spin–flip process in the O2 molecules, which is achieved via electron transfer from Pd surface to O2. With the capability of tuning O2 activation, we have been able to further implement the {100}-faceted nanocubes in glucose oxidation. It is anticipated that t...

In Situ FT-IR Spectroscopic Study of CO2 and CO Adsorption on Y2O3, ZrO2, and Yttria-Stabilized ZrO2.

Abstract: In situ FT-IR spectroscopy was exploited to study the adsorption of CO2 and CO on commercially available yttria-stabilized ZrO2 (8 mol % Y, YSZ-8), Y2O3, and ZrO2. All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO2. Both Y2O3 and YSZ-8 show a much higher reactivity toward CO and CO2 adsorption than ZrO2 because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO2 adsorption on Y2O3 and YSZ-8, which are much more strongly bound on the former, due to formation of higher-coordinated polydentate carbonate species upon annealing. As the crucial factor governing the formation of carbonates, the presence of reactive (basic) surface hydroxyl groups on Y-centers was identified. Therefore, chemisorption of CO2 most likely includes insertion of the CO2 molecule into a reactive surface hydroxyl group and the subsequent formation of a bicarbonate species. Formate formation fo...

CO Chemisorption and Dissociation at High Coverages during CO Hydrogenation on Ru Catalysts

Abstract: Density functional theory (DFT) and infrared spectroscopy results are combined with mechanism-based rate equations to assess the structure and thermodynamics of chemisorbed CO (CO*) and its activation during Fischer–Tropsch synthesis (FTS). CO* binding becomes weaker with increasing coverage on Ru(0001) and Ru201 clusters, but such decreases in binding energy occur at higher coverages on Ru201 clusters than on Ru(0001) surfaces (CO*/Ru = 1.55 to 0.75); such differences appear to reflect weaker repulsive interactions on the curved surfaces prevalent on small Ru201 clusters. Ru201 clusters achieve stable supramonolayer coverages (CO*/Ru > 1) by forming geminal dicarbonyls at low-coordination corner/edge atoms. CO* infrared spectra on Ru/SiO2 (∼7 nm diameter) detect mobile adlayers that anneal into denser structures at saturation. Mechanism-based FTS rate equations give activation energies that reflect the CO*-saturated surfaces prevalent during catalysis. DFT-derived barriers show that CO* predominantly rea...

Thermochemical evidence for strong iodine chemisorption by ZIF-8

Abstract: For the first time, using aqueous solution calorimetry, we clearly identify the chemisorption of an unusually strong iodine charge-transfer (CT) complex within the cages of a metal–organic framework. Specifically, we studied the sorption of iodine gas in zeolitic imidazolate framework-8 (ZIF-8, Zn(2-methylimidazolate)2). Two iodine-loaded ZIF-8 samples were examined. The first, before thermal treatment, contained 0.17 I2/Zn on the surface and 0.59 I2/Zn inside the cage. The second sample was thermally treated, leaving only cage-confined iodine, 0.59 I2/Zn. The energetics of iodine confinement per I2 (relative to solid I2) in ZIF-8 are ΔHads = −41.47 ± 2.03 kJ/(mol I2) within the cage and ΔHads = −18.06 ± 0.62 kJ/(mol I2) for surface-bound iodine. The cage-confined iodine exhibits a 3-fold increase in binding energy over CT complexes on various organic adsorbents, which show only moderate exothermic heats of binding, from −5 to −15 kJ/(mol I2). The ZIF-8 cage geometry allows each iodine atom to form two CT...

Real-Time Observation of Surface Bond Breaking with an X-ray Laser

Abstract: We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells—chemisorbed and precursor state separated by an entropy barrier—explain the anomalously high prefactors often observed in desorption of molecules from metals.

Catalytic Properties of Transition Metal–N4 Moieties in Graphene for the Oxygen Reduction Reaction: Evidence of Spin-Dependent Mechanisms

Abstract: The O2 dissociation after the chemisorption on the metal center of M–N4 moieties in graphene (with M = Mn, Fe, and Co) is addressed by density functional theory calculations. Both minimum energy paths and saddle points for the oxygen reduction reaction (ORR) in the allowed spin states have been identified. Our calculations indicate that ORR can evolve through different spin states, those where the M–O2 adducts are stable. We find that Mn–N4 and Fe–N4 centers in graphene exhibit the lowest O2 dissociation energies of ∼0.7 and 1.1 eV, respectively, over three spin channels, while for Co–N4 we find two spin channels with the same dissociation energy of ∼1.6 eV. The O2 dissociation barriers on the Mn–N4 and Fe–N4 centers are comparable to that found on Pt(111), suggesting similar ORR catalytic activity, in agreement with experimental results.

CO2 Adsorption on Cu2O(111): A DFT+U and DFT-D Study

Abstract: Adsorption of CO2 on the Cu2O(111) surface is investigated using density functional theory + U with and without dispersion corrections. A number of adsorbate geometries are considered on four different surface terminations that include the bulk-terminated surface and surfaces with oxygen and/or copper vacancies. CO2 is found to adsorb most strongly as a tilted linear molecule coordinated to an unsaturated surface cation. Surface vacancies allow for bent adsorbate configurations to be accessed but they are all less stable than the linear adsorbate. Bader analysis confirms that adsorption of bent CO2 is accompanied by charge transfer from the surface to the molecule, whereas minimal charge transfer occurs in linear physisorption. We show that surface oxygen vacancies have a small impact on adsorption free energies, while surface copper vacancies result in a significant reduction of CO2 adsorption. Including dispersion corrections increases the stability of adsorbed CO2, but adsorption is mostly endoergic at...

Composition-dependent morphostructural properties of Ni-Cu oxide nanoparticles confined within the channels of ordered mesoporous SBA-15 silica.

Abstract: NiO and NiO-CuO polycrystalline rodlike nanoparticles were confined and stabilized within the channels of ordered mesoporous SBA-15 silica by a simple and viable approach consisting in incipient wetness impregnation of the calcined support with aqueous solutions of metal nitrates followed by a mild drying step at 25 °C and calcination. As revealed by low- and high-angle XRD, N2 adsorption/desorption, HRTEM/EDXS and H2 TPR analyses, the morphostructural properties of NiO-CuO nanoparticles can be controlled by adjusting their chemical composition, creating the prerequisites to obtain high performance bimetallic catalysts. Experimental evidence by in situ XRD monitoring during the thermoprogrammed reduction indicates that the confined NiO-CuO nanoparticles evolve into thermostable and well-dispersed Ni-Cu heterostructures. The strong Cu-Ni and Ni-support interactions demonstrated by TPR and XPS were put forward to explain the formation of these new bimetallic structures. The optimal Ni-Cu/SBA-15 catalyst (i.e., Cu/(Cu+Ni) atomic ratio of 0.2) proved a greatly enhanced reducibility and H2 chemisorption capacity, and an improved activity in the hydrogenation of cinnamaldehyde, as compared with the monometallic Ni/SBA-15 or Cu/SBA-15 counterparts, which can be associated with the synergism between nickel and copper and high dispersion of active components on the SBA-15 host. The unique structure and controllable properties of both oxidic and metallic forms of Ni-Cu/SBA-15 materials make them very attractive for both fundamental research and practical catalytic applications.

Kinetic study and the effect of particle size on low temperature CO oxidation over Pt/TiO2 catalysts

Abstract: A series of Pt/TiO2 catalysts with various Pt particle sizes were prepared and tested for low temperature CO oxidation. The effect of Pt particle sizes on the reaction was investigated. It was found that turnover frequencies based on Pt dispersion varied as declined with increasing Pt particle size as a function of d−0.86. However, turnover frequency based on Pt atoms located on the periphery of Pt–TiO2 interface remained constant at 40 °C, implying that these periphery Pt atoms were the active sites. Kinetic study was conducted to investigate reaction pathway on the catalyst. The derived power rate law expression was r = 1.98 x 10−7 PCO0.29PO20.19 at 40 °C. Based on the kinetic results, elementary steps of this reaction were proposed, which involved chemisorption of CO on Pt atoms and chemisorption of O2 on TiO2 and a reaction of these two species at the Pt–TiO2 interface.

The strategies for improving carbon dioxide chemisorption by functionalized ionic liquids

Abstract: CO2 capture and storage (CCS) has attracted worldwide interest because CO2 makes a significant contribution to global warming and climate change as a major greenhouse gas. Ionic liquids are promising absorbents for carbon capture due to their negligible vapour pressures, wide liquid range, high thermal stabilities and tunable properties. This perspective review focuses on the recent advances in chemical absorption of CO2 using functionalized ionic liquids, including amino acid-based ionic liquids, azole-based ionic liquids, phenol-based ionic liquids, and so on. Strategies for how to enhance CO2 absorption capacity, reduce CO2 absorption enthalpy, and improve CO2 absorption kinetics are presented. The absorption mechanisms of these functionalized ionic liquids are explained at the molecular level through a combination of theoretical calculation and spectroscopic investigation. Particular attention is paid to the latest developments in carbon capture by non-amino anion-functionalized ionic liquids. In the last section, future directions and prospects for CO2 capture by functionalized ionic liquids are outlined.

Poly(N-isopropylacrylamide-co-acrylic acid) hydrogels for copper ion adsorption: Equilibrium isotherms, kinetic and thermodynamic studies

Abstract: Hydrogel was successfully synthesised by incorporating acrylic acid (AA) as copper ion (Cu 2+ ) chelator into the thermo-responsive poly( N -isopropylacrylamide) (PNIPAM) via emulsion polymerisation. The stimuli-responsive properties and the Cu 2+ adsorption capacity of the cross-linked PNIPAM-co-AA hydrogels were evaluated. The hydrodynamic diameter of the PNIPAM-co-AA hydrogel particles was studied using dynamic light scattering (DLS) method. It was found that the volume phase transition temperature (VPTT) of hydrogels shifted from 32 °C to 27 °C after Cu 2+ adsorption and the hydrodynamic diameter decreased with increasing temperature. The size of the PNIPAM-co-AA hydrogel particles and the incorporation of AA functional group were confirmed by transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR), respectively. Equilibrium isotherms, kinetic and thermodynamic studies have also been evaluated for its copper ions adsorption. The adsorption capacity ( q m ) of PNIPAM-co-AA hydrogels for Cu 2+ was found to be 67.25 mg g −1 with best fit to Langmuir isotherm and the adsorption mechanism follows the pseudo-second-order model. Gibbs free energy analysis shows that the adsorption was spontaneous and it exhibited endothermic chemisorption properties. Adsorption of copper ions on PNIPAM-co-AA hydrogels was found to be more thermodynamically driven.

Interfacial Properties of Bilayer and Trilayer Graphene on Metal Substrates

Abstract: One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni and Co substrates is a strong chemisorption and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact.

The dissociative chemisorption of methane on Ni(111): The effects of molecular vibration and lattice motion

Abstract: We examine the dissociative chemisorption of methane on a Ni(111) surface, using a fully quantum approach based on the Reaction Path Hamiltonian that includes all 15 molecular degrees of freedom and the effects of lattice motion. The potential energy surface and all parameters in our model are computed from first principles. Vibrational excitation of the molecule is shown to significantly enhance the reaction probability, and the efficacy for this is explained in terms of the vibrationally non-adiabatic couplings, vibrational mode softening, and mode symmetry. Agreement with experimental data for molecules initially in the ground and 1ν3 state is good, and including lattice anharmonicity further improves our results. The variation of the dissociation probability with substrate temperature is well reproduced by the model, and is shown to result primarily from changes in the dissociation barrier height with lattice motion. The enhancement of dissociative sticking with substrate temperature is particularly s...

Adsorption kinetics of some textile dyes onto granular activated carbon

Abstract: Adsorption kinetics of Acid Blue 113 (AB), Basic Red 5 (BR) and Reactive Yellow 81 (RY) textile dyes were studied. The investigations were essentially conducted to determine the influence of the initial dye concentration of aqueous solutions on the adsorption kinetics. The experiments, carried out in a batch reactor, allowed the determination of the equilibrium times (≤1 h for RY and AB and ≈4 h for BR). The experimental data were analyzed according to the surface reaction and intra-particle models. In effect, the adsorption results were perfectly fitted to the pseudo-second order, with very high regression coefficients ( r 2 ), predicting a significant intra-particle diffusion stage in the controlling of the adsorption process. Moreover, the fitting to Elovich equation, confirms that the process is ensured by chemisorption on a highly heterogeneous material.

Crystal‐Plane‐Controlled Surface Chemistry and Catalytic Performance of Surfactant‐Free Cu2O Nanocrystals

Abstract: Surfactant-free Cu2 O nanocrystals, including cubes exposing {100} crystal planes, octahedra exposing {111} crystal planes, and rhombic dodecahedra exposing {110} crystal planes, were used as model catalysts to study the effect of the crystal plane on the surface chemistry and catalytic performance for CO oxidation of Cu2 O nanocrystals The catalytic performance follows the order of octahedra rhombic dodecahedra>cubes; this suggests that Cu2 O(111) is most active in catalyzing CO oxidation among Cu2 O (111), (110), and (100) surfaces CO temperature-programmed reduction results demonstrate that Cu2 O octahedra are the most easily reduced of the Cu2 O cubes, octahedra, and rhombic dodecahedra Diffuse reflectance FTIR spectra show that CO chemisorption on Cu2 O nanocrystals depends on their shape and the chemisorption temperature CO chemisorption is strongest on rhombic dodecahedra at 30°C, but at 150°C on octahedra Both the reducibility and chemisorption ability of various Cu2 O nanocrystals toward CO are consistent with their catalytic performance in CO oxidation The observed surface chemistry and catalytic performance in CO oxidation of various Cu2 O nanocrystals can be well correlated with their exposed crystal plane and surface composition/structure Cu2 O octahedra expose the {111} crystal plane with coordinated, unsaturated Cu(I) sites, and thus, are most active in chemisorbing CO and catalyzing CO oxidation These results nicely demonstrate the crystal-plane-controlled surface chemistry and catalytic performance of oxide catalysts

Finding Correlations of the Oxygen Reduction Reaction Activity of Transition Metal Catalysts with Parameters Obtained from Quantum Mechanics

Abstract: To facilitate a less empirical approach to developing improved catalysts, it is important to correlate catalytic performance to surrogate properties that can be measured or predicted accurately and quickly, allowing experimental synthesis and testing of catalysts to focus on the most promising cases. Particularly hopeful is correlating catalysis performance to the electronic density of states (DOS). Indeed, there has been success in using just the center of the d-electron density, which in some cases correlates linearly with oxygen atom chemisorption energy, leading to a volcano plot for catalytic performance versus “d-band center”. To test such concepts we calculated the barriers and binding energies for the various reactions and intermediates involved in the oxygen reduction reaction (ORR) for all 12 transition metals in groups 8–11 (Fe–Cu columns). Our results show that the oxygen binding energy can serve as a useful parameter in describing the catalytic activity for pure metals, but it does not necess...

Sulfur dioxide adsorbed on graphene and heteroatom-doped graphene: a first-principles study

Abstract: The adsorption of sulfur dioxide (SO2) on intrinsic graphene and heteroatom-doped (B, N, Al, Si, Cr, Mn, Ag, Au, and Pt) graphene samples was theoretically studied using first-principles approach based on density functional theory to exploit their potential applications as SO2 gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. SO2 molecule is adsorbed weakly on intrinsic graphene, and B-, N-doped graphene; in general, strong chemisorption is observed on Al-, Si-, Cr-, Mn-, Ag-, Au-, and Pt-doped graphene. The adsorption mechanisms are discussed from charge transfers and density of states. This work reveals that the sensitivity of graphene-based chemical gas sensors for SO2 can be drastically improved by introducing appropriate dopant, and Cr, as well as Mn, may be the best choices among all the dopants.

Dramatic Effects of Gallium Promotion on Methanol Steam Reforming Cu–ZnO Catalyst for Hydrogen Production: Formation of 5 Å Copper Clusters from Cu–ZnGaOx

Abstract: A new class of copper, zinc, and gallium mixed oxides (CuZnGaOx) with different chemical compositions obtained by a coprecipitation technique is identified as a highly active catalyst for the low-temperature, direct steam reforming of methanol to supply hydrogen gas to portable fuel cell devices. Their catalytic activity and selectivity are found to be critically dependent on the copper surface area, catalyst structure, and metal–support interaction, etc. As a result, temperature-programmed reduction has been used to investigate the copper ion reducibility and resulting copper speciation; N2O chemisorption and advanced microscopies to determine specific copper surface area, dispersion, and particle size; XRD to investigate the catalyst structure; EPR spectroscopy to probe the environment of Cu2+ species; and AC impedance spectroscopy to probe the mobility of trapped ions in solids. It is proposed that Ga incorporation into Cu–Zn oxide leads to the formation of a nonstoichiometric cubic spinel phase contai...

Understanding the impact of aluminum oxide binder on Ni/HZSM-5 for phenol hydrodeoxygenation

Abstract: The properties of supported Ni particles on HZMS-5 and Al2O3-HZSM-5 were comparably investigated by diverse characteristic techniques. Ni/Al2O3-HZSM-5 had at least three times higher concentrations of accessible Ni atoms (average diameter Ni-0: 8.8 nm) compared to Ni/HZSM-5 (average diameter Ni-0 : 35 run), which are consistently evidenced by TEM and XRD as well as H-2 chemisorption and IR spectra of adsorbed CO. The Ni nanoparticles interacted strongly with the binder through the interaction between NiO and Al2O3, explored by the combined extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and H-2 temperature-programmed reduction (TPR) techniques. The Bronsted acid sites on two supports probed by IR of adsorbed pyridine were similar, but Lewis acid sites contributed by the gamma-Al2O3 were more abundant on Al2O3-HZSM-5. The acid sites of the two catalysts responded differently to metal incorporation and subsequent treatments, reflecting changes in Al environments illuminated by Al-27 MAS NMR. In situ IR spectra of adsorbed species demonstrates that Al2O3-HZSM-5 has higher adsorption capacity for phenol, cyclohexanone, and cyclohexanol due to stronger adsorption of these compounds on the gamma-Al2O3 binder. (c) 2012 Elsevier B.V. All rights reserved.