Co-adsorbed molecules

The surface composition and availability of certain adsorption sites are not the only factors that determine how CO binds to the surface rather, interactions between CO and co-adsorbed molecules also play an important part. The RAIRS study conducted by Raval et al. [35] showed how NO forces CO to leave its favored binding site on palladium (see Fig. 8.10). When only CO is present, it occupies the twofold bridge site, as the infrared frequency of about 1930 cm-1 indicates. However, if NO is co-adsorbed, then CO leaves the twofold site and ultimately appears in a linear mode with a frequency of approximately 2070 cm-1. Raval and colleagues [35] attributed the move of adsorbed CO to the top sites to the electrostatic repulsion between negatively charged NO and CO, which decreases the back-donation of electrons from the substrate into the In orbitals of CO. In this interpretation, NO has the opposite effect that a potassium promoter would have (see Chapter 9 and the Appendix). 

Figure 3. The different contribution to the Hartree-Fock vibrational shift of chemisorbed CO. FO stands for the Pauli repulsion or Frozen Orbital, FO, contribution, "pol Pt " and "don Pt " correspond to the substrate polarization and donation from the subtrate to the CO adsorbed molecule (ji-backdonation), "pol CO" and "don CO" stand for to the CO polarization and donation from CO to the surface (o-donation), SCF contains the rest of contributions due to coupling between the different mechanisms.

In the context of this chapter, STM data of (co-)adsorbed molecules under UHV and at elevated pressures provide complementary information. When available, such investigations were included here and are briefly discussed. Applications of STM in related fields are numerous one fascinating example is single-molecule spectroscopy by inelastic tunneling spectroscopy (lETS) (223-225). 

Figure 17 illustrates the use of this concept for the interpretation of the influence of co-adsorbed, less mobile molecules (benzene, ethylene) on the mobility of a highly mobile species (methane) in zeolite Na-Y. It turns out that the reduction in the methane mobility with an increasing amount of coadsorbed molecules [159] may be satisfactorily explained by the assumption that any co-adsorbed molecule reduces the transition rate through a particu-... 

Fig. 17 Intracrystalline self-diffusivity of methane ( 2 molecules per supercage, at 25 °C) as a function of the amount of co-adsorbed molecules per window . The solid lines are predictions based on the effective medium approximation of percolation theory with / denoting the ratio of the transition rates through blocked and open windows. From [158] with permission...

The appearance of the Vroman effect and related phenomena can be inhibited by the roughness and the porosity of a surface or due to the formation of hybrid aggregates with macromolecules and oxide NP (Figure 6.41). All these structural features of the interfaces cause reduction of accessibility of pre-adsorbed macromolecules for co-adsorbate molecules as well as for solvent molecules. The confinement effects in restricted space of pores or surface roughness (valleys, Figure 6.34) diminish the mobility of the adsorbed molecules. Therefore, the possibility of the displacement of these molecules by other molecules (even of a larger size) decreases. 

Unlike hard templates (alumina, zeolites, etc.) which require many synthetic steps, surfactant templates may be a convenient alternative. The morphology of PANI and PPy (spheres, wires, flat films) can be modulated though the use of adsorbed surfactants aided by co-adsorbing molecules aligned nanowires of PANI produced by this template assisted method can be self-assembled over large areas for the improvement of microelectronic and sensor devices, as depicted in Fig. 1.11 [139]. 

Molecular sieves have had increasing use in the dehydration of cracked gases in ethylene plants before low temperature fractionation for olefin production. The Type 3A molecular sieve is size-selective for water molecules and does not co-adsorb the olefin molecules. 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

Additional applications of the transfer matrix method to adsorption and desorption kinetics deal with other molecules on low index metal surfaces [40-46], multilayers [47-49], multi-site stepped surfaces [50], and co-adsorbates [51-55]. A similar approach has been used to study electrochemical systems. 

Sketch plausible transition states for (a) the dissociation of a molecule in the gas phase (b) the reaction of cyclopropane to give propene (c) the isomerization of CH3CN to CH3NC (d) the desorption of an atom from a surface (e) the dissociation of an adsorbed molecule such as CO on a metal surface. 

Another way to monitor the expected changes in the metal electronic structure is to look at the adsorbed molecules, which are sensitive in their properties to the changes in the electronic structure of surface metal atoms. Such a molecule is CO and the frequency of the CO stretch vibrations ( v(CO)) is a sensitive detector of the direct- and back-donation upon adsorption of CO. It has been reported, that v(CO) decreases for the VIII group metal by alloying of Pd with Ag (22), Ni with Cu (23), but also when mixing Ni with Co (24). This has been first explained (25) as an indication for an increased backdonation due to an assumed electron shift Cu Pt,... 

Adsorbed CO layers, bonding and Interactions, 559-61 Adsorbed molecules, vibrational analysis, 392-V03 Adsorbed species and processes on surfaces, IR spectroscopic characterizations, VOV-19 Adsorption... 

Fundamental is that the atoms in the surface pha.se are not fully co-ordinated. These sites are often called Co-ordinatively Unsaturated Sites (CUS) . These sites chemisorb molecules because upon formation of bonds with the adsorbing molecules the Gibbs free energy is lowered. 

In calculating the metallic surface area, one has to take proper care of the reaction stoichiometry. In the ideal case, a molecule occupies one site, as shown for terminal adsorbed CO in Fig. 3.46.a. Alternatively, a molecule may chemisorb on more than one metal atom, as shown in Fig. 3.46.b and c for bridged-site adsorbed CO and in Fig. 3.46.d for valley-site adsorbed CO, respectively. In some specific cases of really big molecules, one can imagine that a molecule adsorbs on only one site, while simultaneously blocking adjacent sites for geometric reasons. In case an adsorbate molecule adsorbs dissociatively, it will occupy more than one site as shown in Fig. 3.46.e. 

Overall, we demonstrated electrode potential- and time-dependent properties of the atop CO adsorbate generated from the formic acid decomposition process at three potentials, and addressed the issues of formic acid reactivity and poisoning [Samjeske and Osawa, 2005 Chen et al., 2003,2006]. There is also a consistency with the previous kinetic data obtained by electrochemical methods the maximum in formic acid decomposition rates was obtained at —0.025 V vs. Ag/AgCl or 0.25 V vs. RHE (cf. Fig. 12.7 in [Lu et al., 1999]). However, the exact path towards the CO formation is not clear, as the main reaction is the oxidation of the HCOOH molecule ... 

Cho M, Hess C, Bonn M. 2002. Lateral interactions between adsorbed molecules Investigations of CO on Ru(OOl) using nonlinear surface vibrational spectroscopies. Phys Rev B 65 ... 

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