Admolecules

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

The specific interaction of the admolecule with the surface is then rather well established, while the geometry of the adsorbed species is only tentative. One important conclusion to be drawn from the study of the chemical shifts, is that they cannot by themselves indicate unambiguously the exact geometry of a "contact-type complex". Nevertheless the 7r-complex nature of the adsorbed species was also suggested by the dependence of the adsorption coefficient of n-butenes on their energy of ionization (4). 

The nature of the adsorbed species can be inferred from the usual chemical parameters, i.e. chemical shifts, linewidths and relaxation times. These latter allow the study of the mobility on the surfaces. As an analytical tool, C-NMR spectroscopy can also be used to determine the concentration of reactants or products as a function of time and hence kinetic constants can easily be determined. As a conclusion, a rather complete kinetic study can be carried out involving the nature of interaction between the admolecule and the surface and eventually the nature of the surface active centers. One can finally arrive at the proposition of a reaction mechanism. 

Finally we address an STM study providing direct insight into the formation of coordination compounds at a Cu(100) surface, whereby translational and rotational molecular motions are involved. Towards this goal a molecular building block — 1,3,5-benzenetricarboxylic acid (trimesic acid, tma) — was deposited on the copper substrate. At room temperature the carboxylic acid moieties deprotonute and the resulting trimesate admolecules bind flat on... 

It should be stressed that Eqs. (9) and (10a) contain some neglected negative terms, so that QABji can change nonmonotonically with n. In general, admolecules with larger values of DAB prefer lower coordination sites, as will be discussed in detail in Section III,A. 

In the zero-coverage extreme, the BOC-MP analytic results are exact for atomic A adsorbates and are well defined for diatomic AB adsorbates. Formally, Eqs. (9)—(14) can be applied to polyatomic admolecules provided they are treated as quasidiatomics—in other words, if we define the effective parameters QA, QB, and DAB for the relevant molecular fragments A and B. Actually, we have already done such partitioning while extending Eqs. (11)—(14) to Eqs. (17)-(20). Now let us discuss the partitioning problem in a more systematic way. 

Consider first those applications of Eqs. (9)—(14) to polyatomic admolecules AB when bond energy partitioning does not affect the results. The simplest case is monocoordination tj M -AB via an atom A, where B is a set of atoms directly bound to A, for example, as in HCO (la), NH3 (II), or CHX (Illa-c) ... 

The easiest test concerns Eq. (22a) describing the LJ-type dissociation. The equation establishes the linear correlation between the dissociation barrier A for a homonuclear admolecule A2 and the atomic heat of chemisorption Qh with the slope of k = 3/2. As seen from Table V, for H2, 02, and N2 dissociated on surfaces of metals as varied as Fe, Ni, Cu, W, and Pt, the experimental values of k lie within the range k = 1.4-1.7, that is, within 10-15% of the theoretical LJ value of k = 1.5. It should be stressed that, unlike similar linear relations between the activation barriers and the heats of reactions (Brpnsted, Polanyi, Frumkin-Temkin-Semye-nov, etc.), Eq. (22a) is not a postulate but a corollary of the general principle (BOC-MP) applied to the one-dimensional dissociation ABS As + Bs. 

Figure 4 Dependence of the initial sticking coefficient vs. crystal temperature for Ag(l 0 0) and Ag(l 1 0). The curves are normalised to the value at T = 100 K for an easier comparison. For Ag( l 0 0) desorption prevails and S0 drops above T = 170 K, when the lifetime of the admolecules becomes shorter than the time constant of the uptake experiment (typically 0.3 sec). On Ag(l 1 0), on the contrary, S0 decreases smoothly with T, suggesting that dissociation must take place also at terrace sites. The values of the initial sticking coefficient of 02/Ag(l 00), estimated from the intensity of the 0/Ag(l 00) vibration at 30meV, [71], is reported in the inset for 200 K < T < 400 K. The increase with 74 s attributed to the thermal generation of active sites, identified with kinks at closed packed steps.

Ethylene molecules are known to physisorb at low crystal temperature. The binding energy in this state was estimated to be 0.25 eV from isothermal desorption experiments on Ag(l 0 0) [84]. Near edge X-rays absorption fine structure showed that the admolecules occupy the fourfold hollows on Ag(l 0 0) with the axis parallel to the surface [85, 86]. The sticking probability into the physisorption well is inhibited for rotationally excited gas-phase molecules [84]. 

In Fig. 5 we compare HREEL spectra recorded after exposing the flat and stepped Ag surfaces at T = 105 K to small amounts of 02 dosed with E[ = 0.39 eY and at a crystal temperature T = 105 K. The angle of incidence was chosen normal to the crystal for Ag(l 0 0) and nearly normal to the (1 1 0) nanofacets for Ag(4 1 0) and Ag(2 1 0). HREEL spectra indicate that at this temperature only ad-molecules are observed on Ag(l 00), at least for small exposures. This is witnessed in the HREEL spectra by the loss at 81 meV [55], corresponding to the internal stretch motion of adsorbed 02, and by the absence of intensity in the frequency region of the O/Ag stretch, between 30 meV and 40 meV [62]. On Ag(4 1 0) partial dissociation occurs since two Ag/O stretch losses are present, at 32 meV and at 40meV, while the internal 02 vibration is visible at 84meV [96]. On Ag(2 1 0), on the contrary, only the low frequency losses are present, indicating that the admolecules are unstable [97]. Our first conclusion is therefore that open steps cause 02 dissociation and that this mechanism is very effective on Ag(2 1 0) and less efficient on Ag(4 1 0) where the terraces have a finite width. Also in this latter case,... 

Figure 5 HREEL spectra recorded after small O2 doses at E = 0.39 eV on flat Ag(l 0 0) (bottom spectrum), Ag(41 0) (middle) and Ag(2 1 0) (top) at T = 105 K. O2 is dosed at normal incidence for Ag(l 0 0) and close to the normal to the step heights for the stepped surfaces. The losses in the 30-40 meV region are due to adatom surface vibrations, those at 80-84 meV to the internal stretch mode of the O2 admolecules. It is evident that only molecular adsorption takes place on flat Ag(l 00), while adatoms and admolecules coexist on Ag(41 0) and the final adsorption state is purely dissociative for Ag(2 1 0). The residual intensity at 84meV in the upper spectrum is most probably due to imperfections of the (21 0) staircase leading to larger terraces. We remind that the oxygen dose is expressed in ML of surface atoms, which are therefore referred to the corresponding face density.

Figure 6 HREEL spectra recorded after dosing O2 with Ex = 0.39 eV on Ag(4 1 0) at T = 105 K and parametric in the angle of incidence. Panel A low exposure limit panel B larger exposure (notice the different multiplication factor). The final adsorption state is initially dissociative for all angles of incidence. Admolecules are eventually stabilised and are observed already at relatively low total coverage. The deactivation of the active site for dissociation occurs most probably through the action of oxygen adatoms at step sites. The molecular adsorption site is reached more efficiently for normal incidence on the terraces.

Figure 7 HREEL spectra recorded after O2 adsorption at different Ex on Ag(41 0) at T = 105 K. The supersonic molecular beam hits the surface at 0 = 31°, i.e. normally to the step heights, in all experiments. Only dissociative adsorption is observed at the lowest E as witnessed by the vibration at 40 meV. An additional adatom species (hoy = 32 meV) forms at slightly larger Ex and admolecules are stable for Ex above 0.30 eV.

Figure 11 Temperature dependence of S0 for oxygen adsorbed on Ag(4 1 0) and Ag(21 0), parametric in 0 and in Ex. The corresponding curves recorded at 0 = 0° for Ag(l 1 0) and Ag(l 00) are also reported for comparison. The shape of the curve is determined by the interplay between desorption, which can occur at any sites, and dissociation, which takes place efficiently at (11 0) terraces and at (11 0) like steps. The close similarity of Ag(21 0) and Ag(410) to Ag(l 1 0) is due to the fact that the admolecules can easily reach the active sites for dissociation.

We have reviewed theoretical approaches concerning perfect surface. Defects modify the adsorption. Hydration can strongly perturb the conclusions. If water saturates the adsorption sites, the admolecule should either displace an adsorbed molecule (and has to be more strongly adsorbed than water itself) or adsorb by hydrogen bonding on an ice layer. Such layers are formed on MgO(lOO) surfaces[3, 24, 80]. 

Figure 9. (A) and (B) Schematic illustrations of two possible real-space structures of the Pd(lll)-c(2V3x3)-rect-C6H6 adlayer. In (A), the benzene molecules occupy two-fold bridge sites in (B), the slightly tilted admolecules occupy three-fold hollow sites. (C) Real-space structural model of the Pd(lll)-( 3x3)-C6H6 adlayer all molecules are situated on three-fold hollow sites.

CO to a Pd(l 1 l)-like thick film and a Pd monolayer supported on Ta(110) [30,31], The spectrum for a thick palladium film is in very good agreement with that observed for adsorption of CO on a single-crystal Pd(lll) surface. The features at 11 and 8 eV correspond to emissions fi om the 4o and (In + 5o) levels of CO, respectively [30,31], In the photoemission spectrum for the Pd monolayer the 4o and (In + 5o) peaks of CO appear at higher binding energy than in the ectrum for the Pd(lll)-like film, and there is also an extra shake-up satellite ( s peak) around 13.6 eV. The spectrum for CO on the Pd monolayer matches the ectrum seen for CO on Cu(lll) [30,31], where the bonding interactions between the admolecule and metal substrate are much weaker than on Pd(l 11). 

Sano M, Seimiya Y, Ohno Y, Matsushima T, Tanaka S, Kamada M (1999) Orientation of oxygen admolecules on a stepped platinum(133) surface. Surf Sci 421 386... 

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