Am(III) adsorption on oxides of aluminium and silicon: effects of humic substances,pH, and ionic strength

The adsorption of Am(III) (total concentration 10(-9) mol/l) on alumina, silica, and hematite was studied by a batch technique. The effects of pH, ionic strength, and humic substances on the adsorption of Am(III) on alumina and silica were investigated, and the adsorption isotherms of Am(III) on alumina and silica at different pH values were determined. It was found that compared with the adsorption of Am(III) on alumina, the adsorbability of silica on the basis of mass is less, the relative adsorption rate on silica is slower, the sensitivity of adsorption on silica to ionic strength is less, the dependence of adsorption on silica on pH is gentler, and consequently that the adsorption characteristics of Am(III) on alumina and silica are distinctly different. The negative effect of fulvic acid on the adsorption on silica and the positive effect of humic acid on the adsorption on alumina were found. In contrast to the Am(III) adsorption on alumina and silica, a tremendously high adsorbability of Am(III) on hematite was found. The sequence of adsorbabilities of Am(III) on the basis of mass is Fe2O3 > Al2O3 > SiO2.     

Influence of coadsorbed hydrogen on ethylene adsorption and reaction on a (radical3 x radical3)R30 degrees-Sn/Pt(111) surface alloy

The effect of surface-bound hydrogen adatoms on adsorption, desorption, and reaction of ethylene (CH(2)=CH(2)) on a (radical3 x radical3)R30 degrees-Sn/Pt(111) surface alloy with theta(Sn) = 0.33 was investigated by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed H decreased the saturation coverage of chemisorbed ethylene and less H was required to completely block ethylene chemisorption on this alloy than that on Pt(111). This is also the first report of extensive H site-blocking of ethylene chemisorption on Pt(111). Preadsorbed H also decreased the desorption activation energy of ethylene on the alloy surface. The reaction chemistry of ethylene on this Sn/Pt(111) alloy is dramatically different than on the Pt(111) surface: the H-addition reaction channel taking ethylene to ethane on Pt(111) is totally inhibited on the alloy. This is important information for advancing understanding of the surface chemistry involved in hydrogenation and dehydrogenation catalysis.     

Pt- (and Co-) Promoted Molybdena-Alumina Catalysts: Analysis of the Impregnation Steps

The equilibrium parameters for the adsorption of Mo(VI) on gamma-Al(2)O(3) and of Co(II) and Pt(IV) on MoO(3)/gamma-Al(2)O(3) were determined. The adsorption isotherms were performed from aqueous solutions of the corresponding precursors on two different alumina supports. According to the classification given by Giles, L-type-shaped, subgroup 2, adsorption curves were found for the system Mo on gamma-Al(2)O(3), L-type, subgroup 1, for the Pt on MoO(3)/gamma-Al(2)O(3), and S-type for Co on the MoO(3)/gamma-Al(2)O(3) system. Numerical calculations were carried out for all the isotherms to find the equilibrium parameters. These constants are being used to model the development of Pt, Co, and Mo profiles on MoO(3)/gamma-Al(2)O(3) or gamma-Al(2)O(3) extrudates, respectively, which belong to the new generation of noble-metal-MoO(3)/gamma-Al(2)O(3)-supported catalysts to be used in oil-refining processes. Copyright 2001 Academic Press.     

Adsorption of O2 on tubelike Au24 and Au24- clusters

The nondissociative adsorptions of O(2) on the neutral and anionic Au(24) have been studied using the density functional theory (DFT) in the generalized gradient approximation. Their geometrical structures are optimized by using a combination of the relativistic effective core potential and all-electron potential with scalar relativistic corrections. It is found that the adsorptions of O(2) on the tubelike Au(24) and Au(24) (-) are more stable than it on their space-filled counterparts. Mulliken population analysis shows that the O(2) adsorbed on the tubelike Au(24) and Au(24) (-) got more electrons than on the amorphous ones, which may be a reason why the O(2) can be adsorbed more easily on the former rather than on the latter. Compared with the previous DFT studies of O(2) adsorbed on small Au(n) (n< or =10) clusters, we have shown that the O(2) can also be adsorbed on the neutral even Au(24) with an adsorption energy compatible with that on the small neutral odd gold clusters, but the adsorption energy of O(2) on the anionic Au(24) (-) is lower than that on the small anionic Au(n) with even n. In all the optimized geometrical structures of the O(2)-adsorbed Au(24) and Au(24) (-) clusters, including both tubelike and amorphous ones, we found that O(2) prefers its two O atoms to be attached to two near gold atoms with the least coordination number rather than only one O atom to be attached to one gold atom.     

Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO(2) has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O(2) dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O(2) dissociation on Au (O(2)-->2O(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O(2) on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O(2) can adsorb on the interface of Au/TiO(2) with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O(2) dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O(2)-->CO(2)+O, and CO+O-->CO(2); and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.     

H atom adsorption and diffusion on Si(110)-(1×1) and (2×1) surfaces

We present a periodic density-functional study of hydrogen adsorption and diffusion on the Si(110)-(1×1) and (2×1) surfaces, and identify a local reconstruction that stabilizes the clean Si(110)-(1×1) by 0.51 eV. Hydrogen saturates the dangling bonds of surface Si atoms on both reconstructions and the different structures can be identified from their simulated scanning tunneling microscopy/current image tunneling spectroscopy (STM/CITS) images. Hydrogen diffusion on both reconstructions will proceed preferentially along zigzag rows, in between two adjacent rows. The mobility of the hydrogen atom is higher on the (2×1) reconstruction. Diffusion of a hydrogen vacancy on a monohydride Si(110) surface will proceed along one zigzag row and is slightly more difficult (0.2 eV and 0.6 eV on (1×1) and (2×1), respectively) than hydrogen atom diffusion on the clean surface.     

Adsorption and dissociation of NH3 on clean and hydroxylated TiO2 rutile (110) surfaces: a computational study

The adsorption and dissociation of NH(3) on the clean and hydroxylated TiO(2) rutile (110) surfaces have been investigated by the first-principles calculations. The monodentate adsorbates such as H(3)N-Ti(a), H(2)N-Ti(a), N-Ti(a), H(2)N-O(a), HN-O(a), N-O(a) and H-O(a), as well as the bidentate adsorbate, Ti-N-Ti(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five-fold-coordinated Ti atoms (5c-Ti), namely H(2)N-Ti(a), HN-Ti(a), N-Ti(a) and Ti-N-Ti(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two-fold-coordinated O atoms (2c-O). The enhanced adsorption of NH(x) (x = 1-2) on the 5c-Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NH(x) (x = 1-2) on the two-fold-coordinated O atoms (2c-O) decreasing its basicity. According to potential energy profile, the NH(3) dissociation on the TiO(2) surface is endothermic and the hydroxyl group is found to lower the energetics of H(2)N-Ti(a)+H-O(a) and HN-Ti(a)+2{H-O(a)}, but slightly raise the energetic of Ti-N-Ti(a)+3{H-O(a)} compare to those on the clean surface. However, the dissociation of NH(3) is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH(3) on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.     

First-principles studies of chiral step reconstructions of Cu(100) by adsorbed glycine and alanine

Adsorption of amino acids on Cu(100) is known experimentally to induce surface reconstructions featuring intrinsically chiral Cu(3,1,17) facets, but no information about the geometry of the molecules on these chiral facets is available. We present density-functional theory calculations for the structure of glycine and alanine at moderate coverages on Cu(3,1,17). As might be expected, molecules prefer to bind at the step edges on this surface rather than on the surface's (100)-oriented terraces. The adsorption of enantiopure alanine on Cu(3,1,17) is predicted to be weakly enantiospecific, with S-alanine being more stable on Cu(3,1,17)(S) than R-alanine. By comparing the surface energies of Cu(100) and Cu(3,1,17) in the presence of adsorbed glycine or alanine, our calculations provide insight into the driving force for chiral reconstructions of Cu(100) by amino acids.     

Dispersion-corrected density functional theory calculations of the molecular binding of n-alkanes on Pd(111) and PdO(101)

We investigated the molecular binding of n-alkanes on Pd(111) and PdO(101) using conventional density functional theory (DFT) and the dispersion-corrected DFT-D3 method. In agreement with experimental findings, DFT-D3 predicts that the n-alkane desorption energies scale linearly with the molecule chain length on both surfaces, and that n-alkanes bind more strongly on PdO(101) than on Pd(111). The desorption energies computed using DFT-D3 are slightly higher than the measured values for n-alkanes on Pd(111), though the agreement between computation and experiment is a significant improvement over conventional DFT. The measured desorption energies of n-alkanes on PdO(101) and the energies computed using DFT-D3 agree to within better than 2.5 kJ/mol (< 5%) for chain lengths up to n-butane. The DFT-D3 calculations predict that the molecule-surface dispersion energy for a given n-alkane is similar in magnitude on Pd(111) and PdO(101), and that dative bonding between the alkanes and coordinatively unsaturated Pd atoms is primarily responsible for the enhanced binding of n-alkanes on PdO(101). From analysis of the DFT-D3 results, we estimate that the strength of an alkane η(2)(H, H) interaction on PdO(101) is ~16 kJ/mol, while a single η(1) H-Pd dative bond is worth about 10 kJ/mol.     

Adsorption of atoms on cu surfaces: a density functional theory study

The chemisorption of atoms (H, N, S, O, and C) on Cu surfaces has been systematically studied by the density functional theory generalized gradient approximation method with the slab model. Our calculated results indicate that the orders of the adsorption energy are H < N < S < O < C on Cu(111) and H < N < O < S < C on Cu(110) and Cu(100). Furthermore, the adsorption energies of the given atoms on Cu(100) are larger than those on Cu(111) and Cu(110). The preferred adsorption sites are a 3-fold hollow site on Cu(111) and a 4-fold hollow site on Cu(100), but the preferred adsorption sites on Cu(110) are different for different adatoms. The energy, as well as the geometry, is in good agreement with the experimental and other theoretical data. In addition, this study focuses on the electronic and geometric properties of the metal-atom (M-A) bond to explain the difference in adsorption energies among adatoms. A detailed investigation of the density of states curves explains the nature of the most stable site. Finally, we test the effect of the coverage and find that the surface coverage has no influence on the preferred adsorption sites of the given adatoms on Cu(110) with the exception of hydrogen and oxygen, but has much influence on the value of the adsorption energy.     

Eu(III) adsorption on rutile: Batch experiments and modeling

Eu(III) adsorption on rutile was investigated as a function of contact time, pH, ionic strength and Eu(III) concentration by using a batch experimental method. The effects of carbonate, sulfate, and phosphate were also studied. It was found that the kinetics of Eu(III) adsorption on rutile could be described by a pseudo-second-order model. The adsorption of Eu(III) on rutile is strongly pH-dependent, but relatively insensitive to ionic strength. A double layer model (DLM) with two inner-sphere Eu(III) surface complexes was applied to quantitatively interpret the adsorption of Eu(III) on rutile. There were no apparent effects of carbonate and sulfate on Eu(III) adsorption, whereas the presence of phosphate promoted Eu(III) adsorption on rutile. The surface complexes of Eu(III) on rutile were evidenced by X-ray photoelectron spectroscopy (XPS).     

High antibacterial efficiency of pDMAEMA modified silicon nanowire arrays

Materials of high antibacterial activity based on quaternized poly (2-(dimethylamino ethyl) methacrylate) (pDMAEMA) have been developed. DMAEMA was graft polymerized on silicon nanowire arrays (SiNWAs) by atom transfer radical polymerization (ATRP), and quaternized using benzyl chloride. The graft density on the modified nanowire arrays was much higher than on analogous smooth silicon, leading to higher bacterial adhesion on the nanowire arrays (34.6±0.39×10(6) vs. 5.0±0.15×10(6) cells/cm(2)). Incubation of Escherichia coli on the substrates for 18 h resulted in 95% cell death on the quaternized nanowire material compared to less than 45% on the quaternized smooth silicon. The results suggest that silicon nanowire array modified with quaternized pDMAEMA is a highly effective antibacterial material due to a high density of antibacterial polymer and consequent high bacterial adhesion and killing.     

Understanding the effect of cobalt particle size on Fischer-Tropsch synthesis: surface species and mechanistic studies by SSITKA and kinetic isotope effect

Co/γ-Al(2)O(3) catalysts with particle sizes in the range of 4-15 nm were investigated by isothermal hydrogenation (IH), temperature programmed hydrogenation (TPH), and steady-state isotopic transient kinetic analysis (SSITKA). Kinetic isotope effect experiments were used to probe possible mechanisms on Co/γ-Al(2)O(3) with different particle size. It was found that CO dissociated on Co/γ-Al(2)O(3) catalysts at 210 °C. The total amount of CO(2) formed following the dissociation depends on the cobalt crystal size. O-Co binding energy was found to be highly dependent on the Co metal particle size, whereas similar C-Co binding energy was found on catalysts with different Co particle size. Very strongly bonded carbon and oxygen surface species increased with decreasing particle size and acted as site blocking species in the methanation reaction. SSITKA experiments showed that the intrinsic activity (1/τ(CH(x))) remained constant as the particle size increased from 4 to 15 nm. The number of surface intermediates (N(CH(x))) increased with increasing particle size. The apparent activation energies were found similar for these catalysts, about 85 kJ/mol. D(2)-H(2) switches further confirmed that the particle size did not change the kinetically relevant steps in the reaction. The reactivity of the active sites on the 4 nm particles was the same as those on the 8, 11, and 15 nm particles, and only the number of total available surface active sites was less on the 4 nm particles than on the others.     

Photoinduced graft polymerization of 2-methacryloyloxyethyl phosphorylcholine on polyethylene membrane surface for obtaining blood cell adhesion resistance

Phospholipid polymer, poly[2-methacryloyloxyethyl phosphorylcholine (MPC)], was grafted with polyethylene (PE) membrane using photoinduced polymerization technique to make the membrane resistant to cell adhesion. The water contact angle on the PE membrane grafted with poly(MPC) decreased with an increase in the photopolymerization time. This decrease corresponded to the increase in the amount of poly(MPC) grafted on the PE surface. The same graft polymerization procedure was applied using other hydrophilic monomers, such as acrylamide (AAm), N-vinylpyrrolidone (VPy) and methacryloyl poly(ethylene glycol) (MPEG). These monomers were also polymerized to form grafted chains on the PE membrane, and the grafting was confirmed with X-ray photoelectron spectroscopy. Analysis of amount and distribution of plasma proteins at the plasma-contacting surface of the original and the modified PE membranes were analyzed using immunogold assay. The grafting of poly(MPC) and poly(VPy) on PE membrane reduced the plasma protein adsorption significantly compared with that on the original PE membrane. However, the PE membranes grafted with poly(AAm) or poly(MPEG) did not show any effects on protein adsorption. Platelet adhesion on the original and modified PE membranes from platelet-rich plasma was also examined. A large number of platelets adhered and activated on the original PE membrane. Grafting with poly(AAm) did not suppress platelet adhesion, but grafting with poly(MPC) or poly(VPy) on the PE membrane was effective in preventing platelet adhesion. It is concluded that the introduction of the phosphorylcholine group on the surface could decrease the cell adhesion to substrate polymer.     

WO3/(TiO2-SiO2)催化剂的表面分散状态及催化性能的研究

用气相流动吸附法制备复合载体,用浸渍法制备WO3/(TiO2-S iO2)催化剂.应用LRS和TPR技术研究WO3在复合载体TIO2-S iO2表面的分散状态,发现TiO2在S iO2表面的分散可增强WO3与载体之间的相互作用,提高WO3在载体表面的分散阈值.另外TPR实验证明,TiO2的存在不仅大大改善WO3的分散状态,而且使WO3在TiO2-S iO2的还原温度升高,WO3与载体之间的作用增强.负载于经TiO2调变的S iO2上的催化剂其HDS、HYD、BHD活性高于负载于单纯S iO2上的催化剂的活性.     

Solvent and temperature gradients in separation of synthetic oxyethylene-oxypropylene block (co)polymers using high-temperature liquid chromatography

Chromatographic behavior of synthetic block (co)oligomer samples (EO)n(PO)m(EO)n and (PO)n(EO)m(PO)n with different distribution of propylene oxide (PO) and ethylene oxide (EO) monomer units was investigated on three types of stationary phases on zirconium dioxide support: Zr-PS (polystyrene), Zr-carbon, and Zr-carbon C18. The effects of the distribution and sequence of the oxyethylene and oxypropylene monomer units on the chromatographic retention depend on the type of the stationary phase, but are strongly affected by the organic modifier (methanol or ACN) in aqueous-organic mobile phase. Special attention was focused on the influence of the mobile-phase composition on the separation according to the EO and PO distribution. Zirconia-based columns are stable at elevated temperatures and can be used in high-temperature LC (HTLC); hence, we investigated the temperature effects on the chromatographic behavior up to 90 degrees C. The applications of solvent and temperature gradients were compared on the zirconia stationary phases in the RP mode.     

C-H bond activation over metal oxides: a new insight into the dissociation kinetics from density functional theory

The C-H activation on metal oxides is a fundamental process in chemistry. In this paper, we report a density functional theory study on the process of the C-H activation of CH(4) on Pd(111), Pt(111), Ru(0001), Tc(0001), Cu(111), PdO(001), PdO(110), and PdO(100). A linear relationship between the C-H activation barrier and the chemisorption in the dissociation final state on the metal surfaces is obtained, which is consistent with the work in the literature. However, the relationship is poor on the metal oxide surfaces. Instead, a strong linear correlation between the barrier and the lattice O-H bond strength is found on the oxides. The new linear relationship is analyzed and the physical origin is identified.     

In situ infrared reflection absorption spectroscopy of carbon monoxide adsorbed on Pt(S)-[n(100)x(110)] electrodes

Structural effects on the adsorption of CO have been studied using infrared reflection absorption spectroscopy (IRAS) on Pt(S)-[n(100)x(110)] surfaces (n = 2, 5, 9) that have densely packed kink atoms in the step. Coverage and potential dependence of the IRAS spectra are scrutinized. On-top and bridge-bonded CO are found on all of the surfaces examined. CO is adsorbed on only kink at low coverage (thetaCO < or = 0.2). Adsorbed CO on kink gives an IR band at lower frequency than that on step. CO is adsorbed on both kink and terrace at 0.3 < or = thetaCO. Water is adsorbed on the terrace of Pt(510) n = 5 and Pt(910) n = 9 at low CO coverage, but water is not found on Pt(210) n = 2 of which the first layer is composed of only kink atoms. It is suggested that coadsorbed water on the terrace enhances the activity for the oxidation of adsorbed CO on the kink remarkably.     

环氧丙烷在金属氧化物表面上吸附的IR研究

环氧丙烷(PO)是一种重要的化工原料,可用于合成多种特殊化学品及材料[1].其中在某些化学品合成过程中,经常以均相酸或碱作催化剂,例如在合成有机溶剂丙二醇醚的过程中就用到了矿物酸或苛性碱.虽然均相酸或碱作催化剂有活性高、选择性好等优点,但同时存在产物与催化剂分离、腐蚀和废液处理等多种弊端,因此在一些反应过程中人们正积极探求用符合要求的多相催化法来代替均相催化法.A l2O3、ZnO和MgO分别具有酸性、两性和碱性并已用于多种催化反应中[2,3],本文通过IR光谱法研究了环氧丙烷在MgO,A l2O3和ZnO上的吸附活化态.在这些氧化物中…     

Effects of bulk impurity concentration on the reactivity of metal surface: sticking of hydrogen molecules and atoms to polycrystalline Nb containing oxygen

Nonmetallic impurities segregated onto metal surfaces are able to drastically decrease the chemical reactivity of metals. In the present paper, effects of bulk impurities on the reactivity of metallic surfaces were investigated in a wide temperature range on an example of the sticking of hydrogen molecules and atoms to Nb [polycrystalline, with mainly (100)] containing solute oxygen. At all the investigated surface temperatures, T(S) (300-1400 K), we found the bulk oxygen concentration C(O) to have a strong effect on the integral probability, alpha(H(2) ), of dissociative sticking of H(2) molecules followed by hydrogen solution in the metal lattice: alpha(H(2) ) monotonically decreased by orders of magnitude with increasing C(O) from 0.03 to 1.5 at. %. The sticking coefficient alpha(H(2) ) was found to depend on T(S) but not on the gas temperature. The effect of C(O) on alpha(H(2) ) is explained by the presence of oxygen-free sites (holes in coverage) serving as active centers of the surface reaction in the oxygen monolayer upon Nb. In contrast to H(2) molecules, H atoms were found to stick to, and be dissolved in, oxygen-covered Nb with a probability comparable to 1, depending neither on C(O) nor on T(S). This proves that, unlike H(2) molecules, H atoms do stick to be dissolved mainly through regular surface sites covered by oxygen and not through the holes in coverage.