Molecular adsorbed

Adsorption may in principle occur at all surfaces its magnitude is particularly noticeable when porous solids, which have a high surface area, such as silica gel or charcoal are contacted with gases or liquids. Adsorption processes may involve either simple uni-molecular adsorbate layers or multilayers the forces which bind the adsorbate to the surface may be physical or chemical in nature. 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

Martel R, Avouris Ph and Lyo l-W 1996 Molecularly adsorbed oxygen species on Si(111)-(7 7) STM-induced dissociative attachment studies Science 272 385... 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

Zimdars D, Dadap J I, Eisenthal K B and Heinz T F 1999 Anisotropic orientational motion of molecular adsorbates at the air-water interface J. Chem. Phys. 103 3425-33... 

We start with a non-interacting molecular adsorbate for which the chemical potential is given by... 

The same trends regarding the effect of sulfur have been reported for NO adsorption on Pt(lOO)90 and Rh(100).6 In the case of Pt(100) dissociative adsorption is completely inhibited upon formation of a p(2x2) overlayer at a sulfur coverage equal to 0.25, while the binding strength of molecularly adsorbed NO is lowered by more than 50 kJ/mol, as calculated by analysis of NO TPD data. Due to this complete inhibition of dissociative adsorption, the CO+NO reaction is completely deactivated, although it proceeds easily on sulfur free Pt(100). In the case of Rh(100) a sulfur coverage of only 0.08 suffices to completely inhibit NO dissociation at 300 K. 

Hence, the properties of the molecularly adsorbed N2 cancel as soon as we take kj Ki together, which is the relevant term in the formation of atomic nitrogen. Similarly, but on a much larger scale, partition functions cancel in the term hi Eqs. (51) and (52). Returning to Eq. (59), the factor of two arises because the rate describes the number of nitrogen atoms, whereas the transition state refers to the molecule, which dissociates into two atoms. 

Figure 10.8. Temperature-programmed reaction of NO and CO on two surfaces of rhodium. The initially molecularly adsorbed NO dissociates entirely at relatively low temperatures, but NO does not desorb. Note the difference in selectivity and reactivity between the surfaces on Rh(lOO) most of the CO oxidizes to CO2 and the reaction already...

The mechanism is thought to involve dissociation of hydrogen, which reacts with molecularly adsorbed CO2 to form formate adsorbed on the surface. The adsorbed formate is then further hydrogenated into adsorbed di-oxo-methylene, methoxy, and finally methanol, which then desorbs. The reaction is carried out under conditions where the surface is predominately empty and the oxygen generated by the process is quickly removed as water. Only the forward rate is considered and the process is assumed to go through the following elementary steps ... 

TPD studies tend to depopulate the molecular state sites during the Initial evacuation or carrier gas sweep. Thus, they are only sampling the slow sites which may well be dissociative as we have not examined this frequency region. The presence of an Intermediate molecular adsorbate would allow us to easily rationalize the possible change In observed molecularlty of the FRC results as the temperature was raised In that the dissociative sites would become accessible at the higher temperatures. 

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

The data in Figs. 2 and 3 suggest a reaction which requires a delicate balance between adsorbate coverages, consistent with a Langmuir-Hlnshelwood mechanism. More extensive data of this type (24-27) indicate that molecularly adsorbed ethylene and O2 are the critical species, consistent with the mechanism proposed below. 

We have described the use of FTMS as a detection method for laser desorption of molecular adsorbates from metal surfaces. FTMS provides several important characteristics for these experiments. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

Rao, C.N.R., Kamath, P.V. andYashonath, S. (1982) Molecularly adsorbed oxygen on metals electron spectroscopic studies. Chemical Physics Letters, 88, 13—16. 

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