Chemical adsorption molecular, calculation

A specialized MOPAC computer software package and, in particular, its PM3 quantum-chemical program has been successfully applied in calculations. The results of calculations have shown that both oxygen atoms form bonds with two more active carbon atoms of CP molecular cluster (so-called bridge model of adsorption). The total energy of system after a chemical adsorption at such active atoms is minimal. 

The enhancement of the ballistic transmission through atomic wires upon strong chemical adsorption is supported by ab initio DFT-based calculations [220], and was also observed in the presence of molecular adsorbates [201]. 

The adsorption of textile dyes on natural sorbents was investigated by various HPLC methods. The chemical structures and calculated molecular areas of the dyes are depicted in Fig. 3.83. Because of their different chemical structures and retention characteristics, the dyes were separated in different columns with different mobile phases. Basic blue 41 (BB41) was determined in an octylsilica column (75 X 4.6 mm i.d.). The mobile phase consisted of methanol-KH2P04-Na2HP04 buffer (pH = 5) in volume ratio 1 1. Acid blue... 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

As a typical example of CEDFT calculations, we present in Fig. 1 the capillary condensation isotherm of N2 in a cylindrical pore mimicking the pore channel in MCM-41 mesoporous molecular sieves. The isotherm is presented in co-ordinates adsorption N versus chemical potential p Calculations were performed at 77 K for the internal diameter of 3.3 nm up to the saturation conditions, point H. We used Tarazona s representation of the Helmholtz free energy [6] with the parameters for fluid-fluid and solid-fluid interaction potentials, which were employed in our previous papers [7]. We distinguish three regions on the isotherm. The adsorption branch OC corresponds to consecutive formation of adsorption layers. Note that the sharp transitions between the consecutive layers are not observed in experiments. They are caused by a well-known shortcoming of the model employed, which ignores intrinsic to real... 

The CG picture of the structure of the water network in VPI-5 molecular sieve as reported by the high resolution synchrotron powder diffraction study[7] is shown in Fig. 5. As mentioned earlier, quantum chemical cluster model calculations by MNDO method have also shown that the energetically favourable sites for the adsorption of water is in correspondence with the experimental reports. 

Fig. Ill-13. (a) Plots of molecular density versus distance normal to the interface a is molecular diameter. Upper plot a dielectric liquid. Lower plot as calculated for liquid mercury. (From Ref. 122.) (b) Equilibrium density profiles for atoms A and B in a rare-gas-like mixmre for which o,bb/ o,aa = 0.4 and q,ab is given by Eq. III-56. Atoms A and B have the same a (of Eq. m-46) and the same molecular weight of SO g/mol the solution mole fraction is jcb = 0.047. Note the strong adsorption of B at the interface. [Reprinted with permission from D. J. Lee, M. M. Telo de Gama, and K. E. Gubbins, J. Phys. Chem., 89, 1514 (1985) (Ref. 88). Copyright 1985, American Chemical Society.]...

In the investigations of molecular adsorption reported here our philosophy has been to first determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron diffraction. Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans theorem a comparison with a quantum chemical cluster calculation is then possible, should this be available. All three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we briefly discuss the techniques. The third Section is devoted to three examples of the combined application of NEXAFS and photoemission, whereby the first - C0/Ni(100) - is chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HCOO/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. 

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization cross-section data obtained either empirically or by calculation. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

The sorption data for tnethanol vapor and water vapor are given in Tables I and II, respectively. Column 7 of each table shows the calculated ratio of sorbed gas molecules to molecules of solid at a relative pressure, C = 1. These extrapolated values are deemed significant because they correlate with the chemical composition of the solid. Experimentally, the adsorption of water or methanol vapor is very low on hydrocarbons having polycyclic aromatic structures. For all substances listed, the ratio of sorbed gas to solid (molecular basis) is around 1 or higher except for Golden Yellow G.K. (0.25) and 1-aminoanthraquinone (0.04). Golden Yellow G.K. has a structure composed of... 

However the sample is prepared, we measure 13C spectra of one or more adsorbates on the catalyst, and then need to interpret the spectra to deduce the structure of adsorption complexes or reactive intermediates formed on the catalyst. In many cases the complexes and intermediates formed are unusual and exotic species for which the interpretation of the spectra may be far less than routine. This is where ab initio chemical shift calculations are essential. In diffraction methods, such as x-ray or neutron diffraction, one can more-or-less easily invert the experimental data to yield molecular structure. There is no straightforward relationship between chemical shift data and structure theoretical calculations provide the bridge between experiment and theory. In a typical study, we model the adsorbates on clusters that represent catalyst active sites, using experience and chemical intuition to create our initial structures. 

The SCs at the silica surface readily react with molecular oxygen. This reaction is convenient to monitor optically (Figure 7.23a). Oxygen chemisorption is accompanied by the disappearance of the optical absorption band of SCs. The adsorption of one molecule leads to the decay of one SC. Quantum-chemical calculations show that the three-member cyclic symmetric structure (Figure 7.24) is the most stable product of oxygen addition to the SC [74] ... 

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