Synthesis,characterization and metal chelating properties of silica-physisorbed and chemisorbed-2,5-dioxypiperazine

2,5-Dioxypiperazine (DOPZ) was covalenty bonded as a chelating compound to chloropropyltrimethoxysilane (Si–Cl) for the formation of a new chemisorbed silica [Si–(CH₂)₃–DOPZ]. Physisorbed silica-loaded-2,5-dioxypiperazine [Si–DOPZ] was also synthesized via a physical adsorption approach. Elucidation of the chemisorption and physisorption of 2,5-dioxypiperazine onto silica was confirmed on the basis of 70 eV electron impact mass spectrometric (70 eV EI-MS) mode of ionization via a direct insertion probe (DIP) as a promising technique for providing characteristic fragment ion peaks. The metal probe testing method and elemental analysis were applied to determine the surface coverage values and these were found to be 0.179 and 0.160 mmol g⁻¹ for [Si–(CH₂)₃–DOPZ] and 0.251 mmol g⁻¹ for [Si–DOPZ]. [Si–(CH₂)₃–DOPZ] was characterized by high stability in acidic and buffer solutions, pH 1–7, compared to [Si–DOPZ]. Differential scanning calorimetry studies (DSC) for the modified silica were performed to evaluate the various thermodynamic and kinetic parameters of the thermal degradation processes, and these have been enumerated. The results obtained by both EI-MS and DSC are very similar in many respects. Metal chelation and stoichiometric properties of chemically modified silica were evaluated on the basis of the metal binding capacity, distribution coefficient and separation factor for a series of metal ions. The evaluated results refer to the high metal chelating properties of [Si–(CH₂)₃–DOPZ] for cadmium(II), lead(II) copper(II) and mercury(II). These four [Si–(CH₂)₃–DOPZ]–metal complexes were also synthesized and the identified stoichiometric ratios were found to be 1:2 based on the nitrogen and metal analysis. EI-MS via 70-eV ionization was also used as a potential method for further confirmation of the metal complex formation based on structure and fragmentation elucidation. DSC studies of these four metal complexes were also performed and evaluated.     

Deactivation of Pd particles supported on Nb2O5/Cu3Au(1 0 0): SFG and TPD studies from UHV to 100 mbar

Structural changes that occur on Pd-Nb₂O₅/Cu₃Au(1 0 0) model catalysts upon thermal annealing were followed by sum frequency generation (SFG) and temperature-programmed desorption (TPD) using CO as probe molecule. SFG experiments were performed both under ultrahigh vacuum and mbar pressure. Heating the catalyst to temperatures above 300 K lead to an irreversible 50% decrease in the CO adsorption capacity and modified the remaining adsorption sites. Alterations of the phase between resonant and non-resonant SFG signals upon annealing indicate a change in the electronic structure of the surface, which excludes Pd sintering or migration of Nb₂O₅ over Pd particles to cause the observed effect and rather suggests the formation of “mixed Pd-NbO_x” sites. The same changes in surface properties also occur during CO hydrogenation at 1 bar and high temperature, pointing to an involvement of “mixed Pd-NbO_x” sites in catalytic reactions.     

Studies on the Zetapotential of Calcite/p-Sulfonato-calix[4,8]arenes

Zeta potential of calixarenes has been reported for the first time. The water-soluble calixarenes has been used as dispersion media in solid/liquid interface. p-sulfonato-calix[4]arene (PSC4) and p-sulfonato-calix[8]arene (PSC8) were synthesized and characterized by FTIR, NMR, mass spectrometry, and HPLC techniques. It was proved that the zeta potential is a fast and simple measurement to know the adsorption behavior of sufonato calixarnes on calcite. The chemisorption of p-sulfonato-calix[n]arene was confirmed by shift in iso electric point, adsorption studies and FTIR. The calculated free energy of adsorption value and its sign suggests the chemical interaction between the calcite surface and p-sulfonato calix[4]and[8]arene.     

First-principles investigation of functionalization-defects on silicon surfaces

We present a theoretical study of chemisorption of CHC-CH₂-COOH molecules on the H:Si(1 0 0) surface. We perform simulations for different chemisorbed configurations, attained by reactions through the alkyne tail. We use the periodic slab approximation for the extended surface, within ab initio density functional theory, and analyse results from several different approaches. We conclude that structures composed of single Si-C bridges are very stable, while a previously proposed structure, with a double Si-C-Si bridge, should be metastable on the flat surface, and introduce electron and hole traps in the Si band gap.     

The local adsorption geometry and electronic structure of alanine on Cu{1 1 0}

The adsorption of alanine on Cu{1 1 0} was studied by a combination of near edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Large chemical shifts in the C 1s, N 1s, and O 1s XP spectra were found between the alanine multilayer and the chemisorbed and pseudo-(3 × 2) alaninate layers. From C, N, and O K-shell NEXAFS spectra the tilt angles of the carboxylate group (≈26° in plane with respect to and ≈45° out of plane) and the C-N bond angle with respect to could be determined for the pseudo-(3 × 2) overlayer. Using this information three adsorption geometries could be eliminated from five p(3 × 2) structures which lead to almost identical heats of adsorption in the DFT calculations between 1.40 and 1.47 eV/molecule. Due to the small energy difference between the remaining two structures it is not unlikely that these coexist on the surface at room temperature.     

Island formation in chemisorption 2. Chemisorption kinetics

Influence of “direct” and “indirect” pathways of dissociative chemisorption on the form of kinetic dependences ϕ(MCS) and S(ϕ) was studied by the Monte Carlo method. Langmuir adsorption observed at S_indir/S_dir≤0.1 gradually changes to island-mediated adsorption with an increase of the “indirect” adsorption contribution at 0.1≤S_indir/S_dir≤1.0. At S_indir/S_dir≥1.0 the island-mediated adsorption dominates: large adsorption islands arise and gradually grow.     

Ab initio cluster model approach to the chemisorption of NH3 on Pt(111)

The chemisorption of NH₃ on Pt(111) has been studied using several Pt₁₀ clusters that model different adsorption sites of the Pt(111) surface. Ab initio methods have been used to obtain a theoretical estimate of several spectroscopic features that can be directly compared to experiment. The comparison includes the variation of the difference between the 3a₁ and 1e levels, the vibrational frequency shifts and the order of stability on different surface sites. Chemisorption at the on-top site is predicted to be favoured, the calculated interaction energy appears to be quite close to the experimental estimate, and it is suggested that NH₃ chemisorbs molecularly, in an N-down orientation, with an equilibrium geometry representing a small distortion from the gas-phase molecular geometry and no azimuthal preference, in good agreement with ESDIAD experiments. Constrained space orbital variation (CSOV) analysis of the interaction has also been performed using a Hartree–Fock wave function. This analysis shows that the leading bonding mechanisms are substrate polarisation and charge transfer from ammonia to the surface.