Mo surfaces

The initial step of the adsorption of cyclic sulfides on a Mo(100) surface is also the formation of adsorbed thiolate groups.395-397 Adsorbed alkyl thiolates decompose to adsorbed sulfur, carbon, and hydrogen on the clean Mo surface, but once the surface is deactivated by adsorbed sulfur, alkanes and alkenes evolve from the surface. Tetrahydrothiophene (34) and trimethylene sulfide decompose on Mo(110) to alkanes and alkenes by way of a common intermediate, which is proposed to be a surface thiolate (33). The thiolate undergoes hydrogenation or dehydrogenation, depending on the surface hydrogen concentration (Scheme 4.115).398 399... 

The initial step of the adsorption of thiols on Mo(100) surface is the formation of adsorbed thiolate groups. Phenyl thiolate is formed upon the adsorption of benzenethiol at 120 K on a clean Mo(110) surface.49 The thiolate intermediate subsequently undergoes competing C—S bond hydrogenolysis to form benzene, or C—S and C—H bond scission to form surface benzyne. The adsorption of benzenethiol was also studied on a sulfur-covered Mo surface.50 Phenyl disulfide is formed via S—H bond scission and S-S bond formation. The S-S linkage is oriented perpendicular and the phenyl ring parallel to the surface. 

Mo(OH)6 may still play a role in isotope frachonation if they adsorb because, at equilibrium, the overall fractionation factor for Mo04 Mo-surface necessarily equals the product of the... 

Bimetallic particles with a very narrow size distribution of circa 1.5 nm have been prepared by decarbonylation under H2 at 400 °C of the impregnated Ru5PtC(CO)i6 on carbon black. EXAFS data indicate that a surface segregation of Pt on the fee Ru structure occurs in the bimetallic nanoparticles. Moreover, they undergo reversible oxidation, forming a MO surface and a core of metal [62]. 

After the first theoretical work of Tamm (1932), a series of theoretical papers on surface states were published (for example, Shockley, 1939 Goodwin, 1939 Heine, 1963). However, there has been no experimental evidence of the surface states for more than three decades. In 1966, Swanson and Grouser (1966, 1967) found a substantial deviation of the observed fie Id-emission spectroscopy on W(IOO) and Mo(lOO) from the theoretical prediction based on the Sommerfeld theory of metals. This experimental discovery has motivated a large amount of theoretical and subsequent experimental work in an attempt to explain its nature. After a few years, it became clear that the observed deviation from free-electron behavior of the W and Mo surfaces is an unambiguous exhibition of the surface states, which were predicted some three decades earlier. 

The adsorption and reaction of methanol on metal surfaces has been widely studied (18-34). Methanol has C-0, C-H, and 0-H bonds, serving as one of the simplest systems for the selective activation of chemical bonds. The methoxyl (CH30(a)) species has been considered as an intermediate of the methanol decomposition. On many transition metal surfaces, adsorbed methanol molecules are usually decomposed to H2 and CO, although Ag and Cu are used as catalysts for the conversion of methanol to formaldehyde. The adsorption and reaction of alcohol molecules on Mo surfaces has been studied on the (100) (4) and (110) (35) surfaces. Alcohol molecules are decomposed effectively also on these surfaces. 

Figure 1. Carbon abundance as a function of mass for both components of a close binary system at the onset of mass transfer. The region from Mx=0 to Mr=Mgi=8.1 Mo corresponds to the originally less massive component (gainer), whereas the carbon distribution of the loser is plotted from 8.1 Mo (surface) to 17.1 M (center). The first occurrence of hydrogen depleted layers (Xat<0.7) and the end of the Roche Lobe Overflow are indicated.

Evidence that adsorbed sulfur atoms rather than sulfur molecules are responsible for blocking the adsorption of hydrogen was obtained by Kikuchi and Ishizuka (76). They observed that a clean Mo surface, after preadsorption of S2 vapor at low temperature, could be saturated with H2 at room temperature. However, when the sulfided surface was heated to 773 K in vacuum, only a very small amount of H2 could be adsorbed. 

Heating at the normal metallizing temperature of 1400°C in H2 or N2-H2 environments dissociates the oxide films on Mo surfaces, but the resultant formation of H20 causes the environment to become damp. Similarly, Mn02 is reduced, or Mn is oxidised, to MnO by environments with a H20/H2 ratio of between 4.3xlO-4 and 2.67. (While this ratio is of chemical significance, it is monitored in practice by measurement of a physical parameter, the Dew Point or... 

Dissociative adsorption of CO has been found on a variety of transition metal surfaces. Broden el al. (17) and Nieuwenhuys (14) correlated the tendency for CO, N2, and NO to dissociate with the position of the transition metal in the periodic table the tendency for dissociation increases the further to the left the metal appears in the table, and it decreases from 3d to 5d metals. Furthermore, the borderline for dissociative or molecular adsorption moves to the right in the sequence CO, N2, NO to O2, being the same order as the bond strength in the free molecules. There is sufficient evidence for the proposed correlation. For example, W and Mo surfaces dissociate CO easily al room temperature dissociative adsorption has not been reported lor Pi, Ir, and Pd(III) surfaces, and CO dissociation has been reported to occur on Ni, Co, and Ru at elevated temperatures. Ben-zinger (IS) suggested that the state of adsorption (molecular or dissociative)... 

Chemisorption of oxygen at 195 K has been used in attempts to estimate Mo surface area in reduced catalysts. " Interpretation of these data in terms of Mo surface area has been questioned." ... 

Thiophene HDS Activities have been reported by Bussell et al. over three low Miller index single-crystal surfaces of Mo and four of Re (104-106). The reaction over Mo was insensitive to the surface structure. The Re(0001) surface showed about the same activity as the Mo surfaces, but Re( 1121) was twice as active, and the (1120) and (1010) surfaces were approximately sixfold more active (106). While Mo surfaces were covered with a near monolayer of partially hydrogenated carbon after reaction, the Re was not moderated by a carbon overlayer. The product distribution over Mo(100) was similar to that reported for powder MoS2 catalysts, although the single-crystal surface of pure Mo was much more active (104). Measurements of the rate of hydrogenation of 35S on Mo(100) suggest that sulfur adatoms are not intermediates in thiophene HDS (104). 

Figure 4 shows joint interfaces between resin-derived C-C composite (C-CAT Composites) and Cu-clad-Mo made using Ticusil. Microstructurally sound joints formed but there was some cracking within the C-C composite (Fig. 4a) presumably due to the low inter-laminar shear strength of C-C composites. Ag- and Cu-rich phases formed in the braze matrix with the Ag-rich phase preferentially precipitating onto C-C (point 2, Fig. 4b) and Cu-clad-Mo surface (point 2, Fig. 4c). A small amount of Cu was detected within the composite (point 4, Fig. 4b).

Figure 4. Desorption activation energies for various probe Lewis bases on chemically modified Mo surfaces.

Figure 6.13 shows the adsorption isotherms for PAM on poly Si and Si02 as a function of PAM concentration. Adsorption of PAM on oxide surfaces increases and reaches a plateau level of approximately 0.23 mg/m. However, PAM is scarcely adsorbed on poly Si surfaces. This is driven by the difference in hydrophobicity, which affects the interaction between PAM and each surface. At high pH, the negative site MO of metal oxide (MO) surface bonds with the weakly acidic NH2 function. Therefore, the interaction between SiO of Si02 surface and NH2 group of PAM led to the selective adsorption of PAM on Si02. 

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