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Adsorption Energy (adsorption + energy)
Selected AbstractsInteraction of CO and NO with the spinel CuCr2O4 (100) surface: A DFT studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2008Xiang-Lan Xu Abstract The characteristics of CO and NO molecules at Cu2+ and Cr3+ ion sites on the CuCr2O4 (100) surface have been studied by first principles calculations based on spin-polarized density functional theory (DFT). The calculated results show that adsorption energies for X-down(C, N) adsorption vary in the order: Cu2+ -CO>Cr3+ -NO,Cr3+ -CO>Cu2+ -NO. CO molecules are preferentially adsorbed at Cu sites, whereas NO molecules adsorb favorably at Cu2+ and Cr3+ ion sites. The C-O and N-O stretching frequencies are red-shifted upon adsorption. Combining the analysis of frontier molecular orbitals and Mulliken charge, for CO and NO X-down adsorption systems, the 5, orbitals donate electrons and the 2,* orbitals obtain back-donated electrons. Although for NO with O-down adsorption systems, the NO-2,* orbitals obtain back-donated electrons from substrates without 5,-donation. Coadsorption calculations show the CO/NO mixture adsorb selectively at the Cu2+ion site but simultaneously at the Cr3+ ion site, respectively. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Theoretical study of hydrogen-bonded complexes of benzene with hydrides of astrochemical interestINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2008M. Nait Achour Abstract Post Hartree,Fock and DFT calculations have been performed for studying the possibility for a benzene support to be linked to various hydrides through a quasi Bz···HA bond. Interaction energy of compounds, including CH bonds (CH4, CH3F, CH2O, CHN, CHNO), NH bonds (NH3, NH2F, NHC, NHCO, NH3O), and OH bonds (OH2, OHF, NCOH), were evaluated, taking basis set superposition error (BSSE) and zero point vibrational energy (ZPVE) corrections into account. Numerical convergence of results with respect to the ingredients included at different steps of theory (basis set, DFT functionals, correlation treatments, geometry optimization) was tested mainly on the example of the water adduct and, for comparison, the Bz···H3O+ system containing a cation instead of a neutral molecule. A rather large range of adsorption energies is obtained, from about 1 kcal/mol for methane to more than 6 kcal/mol for cyanic acid, according to the acidic character of the adsorbed species in each family of Bz···HA bonds. Some consequences for astrophysical problems involving PAHs in the interstellar medium are pointed out. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] The relationship between adsorption energies of methyl on metals and the metallic electronic properties: A first-principles DFT studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2005Gui-Chang Wang Abstract A theoretical study of CH3 adsorbed on the (111) surface of some transition and noble metal surfaces M (M = Cu, Ni, Rh, Pt, Pd, Ag, Au) and on the Fe(100) is presented. We find that the hollow site is preferred more than the top one for Fe, Ni, Rh, and Cu, but it is the other way for Pt, Pd, Au, and Ag. In addition, a good linear relationship was observed between the chemisorption energy and d-band center for Group VIII metals or the square of the coupling matrix element for Group IB metals at the hollow site. Interestingly, with a detailed comparison of the adsorption energies at the top and hollow sites, we find that the adsorption energies among each group are very similar on the top site, which supports the theoretical model of Hammer and Nørskov that the coupling between the HOMO of adsorbate and sp states of the metal is dominant and almost equal, and that the second coupling to the d-band contributes less but reflects the change of the adsorption energy. It confirms that the coupling to the d band comprises two opposite factors, that is, the d-band center was attractive and the square of the coupling matrix element was repulsive, such that the contributions from the two factors can counteract each other at the top site. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 871,878, 2005 [source] Adsorption of benzene over the rutile TiO2 (110) surfaces: A theoretical studyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010A. R. R. Neto Abstract In this work, we present our theoretical results for the relaxation of the rutile TiO2 (110), as well as for the adsorption of benzene over this surface. Our results are in good agreement with both the available theoretical and experimental data, whenever these comparisons were possible. Based on our obtained results, we show that the interaction between the adsorbate and the TiO2 (110) surface shows a van der Waalslike character, with the calculated adsorption energies at the order of 43 to 173 meV. In this adsorption process, the H atoms of the benzene prefer to bind with the surface Ti atoms. As a result, two occupied electronic levels appear at the surface bandgap, which are responsible for the optical absorption peaks at the blue-violet region of the electromagnetic spectra. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Surface Composition of Materials Used as Catalysts for Methanol Steam Reforming: A Theoretical Study,CHEMPHYSCHEM, Issue 8 2006Kok Hwa Lim Dr. Abstract PdZn (1:1) alloy is assumed to be the active component of a promising catalyst for methanol steam reforming. Using density functional calculations on periodic supercell slab models, followed by atomistic thermodynamics modeling, we study the chemical composition of the surfaces PdZn(111) and, as a reference, Cu(111) in contact with water and hydrogen at conditions relevant to methanol steam reforming. For the two surfaces, we determine similar maximum adsorption energies for the dissociative adsorption of H2, O2, and the molecular adsorption of H2O. These reactions are calculated to be exothermic by about ,40, ,320, and ,20 kJ,mol,1, respectively. Using a thermodynamic analysis based on theoretically predicted adsorption energies and vibrational frequencies, we determine the most favorable surface compositions for given pressure windows. However, surface energy plots alone cannot provide quantitative information on individual coverages in a system of coupled adsorption reactions. To overcome this limitation, we employ a kinetic model, from which equilibrium surface coverages of H, O, OH, and H2O are derived. We also discuss the sensitivity of our results and the ensuing conclusions with regard to the model surfaces employed and the inaccuracies of our computational method. Our kinetic model predicts surfaces of both materials, PdZn and Cu, to be essentially adsorbate-free already from very low values of the partial pressure of H2. The model surfaces PdZn(111) and Cu(111) are predicted to be free of water-related adsorbates for a partial H2 pressure greater than 10,8 and 10,5 atm, respectively. [source] Molecular dynamics study for dissociation phenomena of a gas molecule on a metal surfaceHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 8 2008Takashi Tokumasu Abstract The dissociation phenomena of a gas molecule on a metal surface were analyzed by the molecular dynamics method. A platinum (111) surface and hydrogen were chosen as the metal surface and the gas molecule, respectively. The embedded atom method was used as the interaction between atoms in order to express the dependence of electron density. The parameters were determined so that the results such as the electron density, adsorption energy of an H atom on a Pt(111) surface, and the interaction between H atoms of an H2 molecule obtained by the EAM method were consistent with those obtained by the density functional theory or empirical function. Collisions between a hydrogen molecule and the platinum surface were simulated by the molecular dynamics method, and the dissociation probability was obtained. Using these results, the effect of the motion of the surface atoms or the hydrogen molecule on the dissociation probability was analyzed. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20222 [source] The relationship between adsorption energies of methyl on metals and the metallic electronic properties: A first-principles DFT studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2005Gui-Chang Wang Abstract A theoretical study of CH3 adsorbed on the (111) surface of some transition and noble metal surfaces M (M = Cu, Ni, Rh, Pt, Pd, Ag, Au) and on the Fe(100) is presented. We find that the hollow site is preferred more than the top one for Fe, Ni, Rh, and Cu, but it is the other way for Pt, Pd, Au, and Ag. In addition, a good linear relationship was observed between the chemisorption energy and d-band center for Group VIII metals or the square of the coupling matrix element for Group IB metals at the hollow site. Interestingly, with a detailed comparison of the adsorption energies at the top and hollow sites, we find that the adsorption energies among each group are very similar on the top site, which supports the theoretical model of Hammer and Nørskov that the coupling between the HOMO of adsorbate and sp states of the metal is dominant and almost equal, and that the second coupling to the d-band contributes less but reflects the change of the adsorption energy. It confirms that the coupling to the d band comprises two opposite factors, that is, the d-band center was attractive and the square of the coupling matrix element was repulsive, such that the contributions from the two factors can counteract each other at the top site. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 871,878, 2005 [source] A simple model for vapor-moisture equilibrium in biomass subtratesAICHE JOURNAL, Issue 6 2009V. R. Vásquez Abstract Equilibrium moisture content (EMC) plays a fundamental role on the properties of bioderived solids such as food, waste biomass, seeds, and many others. For foods, the moisture content affects flavor, texture, and the way the food is stored. In other applications, moisture affects drying conditions and energy requirements for biomass processing. Adequate modeling of EMC in biomass as a function of the water activity is very important in many engineering applications. Most models used to correlate EMC with water activity are based on traditional adsorption models which take into account the adsorption energy of water with the substrate only and do not consider other nonadsorption interactions of water within the substrate such as mixing and swelling effects. Many other models are empirical, which are commonly used for interpolation. Although a detailed description of this type of substrate is very complex, we present a simple model based on standard molecular-thermodynamic models and standard statistical mechanics formulations to correlate the EMC with water activity that attempts to consider both adsorption and nonadsorption interactions of the water with the substrate. Although the model is basic, it captures the behavior of water in this type of systems fairly well and it can be used to fit experimental data with parameters that provide better physical insight on the nature of system. We suggest how to improve the model fundamentally if better knowledge of the water interactions in the substrate are available. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Pore-size effects on activated-carbon capacities for volatile organic compound adsorptionAICHE JOURNAL, Issue 8 2002Mei-Chiung Huang Gas-phase adsorption of acetone and n-hexane by activated carbons with different pore structures was investigated. The carbons were prepared from a bituminous coal with KOH activation. Increasing the activation temperature increased both the porosity and pore size. The equilibrium adsorption capacity for the organic compounds increased with the carbon porosity, but not proportionally. The percentage of pore volume utilized showed a decreasing trend with the porosity development for acetone adsorption, while an increasing trend was observed for n-hexane. By incorporating pore size distribution with the Dubinin-Radushkevich equation using an inverse proportionality between the micropore size and adsorption energy, the isotherms for adsorption onto different carbons can be well predicted. Simulations indicated that the adsorption energy, which is an inverse function of the micropore size, determines the adsorption capacity. Different effects of porosity development were observed for different adsorbates. [source] Adsorption Behavior of Asymmetrical Triblock Copolymers at the Solid-Liquid Interface by Monte Carlo SimulationMACROMOLECULAR THEORY AND SIMULATIONS, Issue 8 2004Changjun Peng Abstract Summary: Monte Carlo simulation on a simple lattice model has been used to study the adsorption of asymmetrical triblock copolymers from a non-selective solvent at the solid-liquid interface. The size distributions of train, loop and tail configurations for those copolymers are obtained as well as other details of the adsorption layer microstructure. Also the influence of adsorption energy and the role of molecular symmetry are investigated. A segment-density profile, the adsorption amount, the surface coverage, and the adsorption layer thickness have been determined. Finally, it is shown that the adsorption behavior of an asymmetrical copolymer can be predicted from the symmetrical copolymer. Size distributions of the tail configuration for A8,kB20Ak. [source] Interface with organic molecules: Cysteine on Au(110)PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Benjamin Höffling Abstract We study the adsorption of the amino acid cysteine on the Au(110) surface by means of density functional theory. Different adsorbate-substrate geometries are investigated regarding geometry, adsorption energy, and changes in the density of states. In particular we compare flat versus upright adsorption configurations in which the molecule registers to the surface via a deprotonized thiolate head group and eventually the amino side group. Sulfur-gold and amino-gold bonding geometries are discussed with their implications for electronic properties (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] CO Adsorption on a LaNi5 Hydrogen Storage Alloy Surface: A Theoretical InvestigationCHEMPHYSCHEM, Issue 11 2008Song Han Dr. Abstract Density functional theory calculations are carried out to study CO adsorption on the (001) surface of a LaNi5 hydrogen storage alloy. At low coverages, CO favors adsorption on NiNi bridge sites. With an increase in CO coverage, the decrease in the adsorption energy is much larger for NiNiCO bridge adsorption than that for NiCO on-top adsorption. Thus, the latter sites in the relatively stable adsorption structure are preferentially utilized at high CO coverages. The nature of the bonding between CO and the LaNi5 (001) surface is analyzed in detail. [source] Theoretical and Experimental Study of the Adsorption of Neutral Glycine on Silica from the Gas PhaseCHEMPHYSCHEM, Issue 6 2005C. Lomenech Dr. Abstract The adsorption of neutral glycine onto amorphous silica was investigated both theoretically and experimentally. DFT calculations were performed at the BLYP-631++G** level using a cluster approach. Several possible configurations involving the formation of H bonds between glycine and one, two, or three silanol groups (SiOH) were considered. The most favorable bonding of glycine with one silanol group (45 kJ,mol,1) occurs through the COOH moiety, thus forming a cycle in which the CO group is an H-bond acceptor whereas the acidic OH group is an H-bond donor. With two or three silanol groups, additional H bonds are formed between the amine moiety and the silanol groups, which leads to an increased adsorption energy (70 and 80 kJ,mol,1for two and three silanol groups, respectively). Calculated ,CO, ,HNH, and ,HCHvalues are sensitive to the adsorption mode. A bathochromic shift of ,COas compared to the ,COof free glycine (calculated in the 1755,1790 cm,1range) is found for glycine in interaction with silanol(s). The more H bonds are formed between the COOH moiety and silanol groups, the higher the bathochromic shift. For ,HNH, no shift is found for glycine adsorbed on one and two silanol groups (where the amine is either not bound or an H-bond donor), whereas a bathochromic shift is calculated with three silanols when the amine moiety is an H-bond acceptor. Experimental FTIR spectra performed at room temperature for glycine adsorbed at 160,°C on Aerosil amorphous silica exhibit bands at 1371, 1423, 1630, and 1699 cm,1. The experimental/calculated frequencies have their best correspondence for glycine adsorbed on two silanol groups. It is important to note that the forms giving the best correspondence to experimental frequencies are the most stable ones. [source] | ||||||||||