Molybdenum oxides, adsorption

In the case of solid or liquid solutions it is frequently observed that one component of the solution is present at a greater concentration in the surface region than in the bulk of the solution. Thus, for an ethanol-water system, die surface region will contain an excess of ethanol. The concentration of water will be higher at the surface than in the bulk, if the solute is sulfuric acid. Molybdenum oxide dissolved in glass will concentrate at the surface of the glass. The concentrating of solute molecules at a surface is called adsorption. 

Krylov (62) studied the adsorption of oxygen and propylene on vanadium oxide/MgO and molybdenum oxide/MgO catalysts by ESR and IR at 25°C. He observed the formation of Qr radicals and ir-allyl complexes during the simultaneous adsorption of 02 and C3H . The data indicated that an electron transfer took place from the olefin to the oxygen through the transition metal ion forming the following complex ... 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

Comparison of the site densities from Table XIX with metal areas determined from H2 adsorption provides important insights into the nature of H2S adsorption on these catalysts. For example, the sulfur site density of 213 /tmol/g compared to the metal site density of 182 /rmol/g (from H2 adsorption) for 14% Ni/Al203 is equivalent to S/Nis = 0.6, in reasonable agreement with the earlier discussed studies (Section III,C) which show values of 0.5-0.8 and consistent with the value 0.6 determined for pure unsupported Ni. However, in the case of a typical molybdenum-containing catalyst, e.g., 10% Ni/20% Mo/A1203, the sulfur site density and H2 uptake are 693 and 72 /imol/g, respectively (S/Nis = 4.8), providing evidence that a considerable amount of sulfur adsorbs on molybdenum oxide sites which do not adsorb H2 a similar behavior is also observed for Raney Ni and nickel-boride catalysts. 

PHOTOLlJMINnSCFNCr, ADSORPTION, AND (PIIOTO)CATALYSIS 195 2. Anchored Molybdenum Oxide... 

In the following, we discuss recent concepts and theoretical results concerning microscopic properties of vanadium and molybdenum oxide surfaces. While the two elements form different classes of oxides their surfaces exhibit numerous stmctural and electronic similarities, such as microscopic surface binding, adsorption, or oxygen vacancies, which we will point out accordingly. 

Atomic and molecular adsorption at molybdenum oxide surfaces have been studied theoretically using both periodic slab and cluster models where so far studies are restricted to the trioxide, M0O3, as a substrate. Further, adsorbate species include in all cases atoms (H [138, 218, 227-229]) or small molecules (methanol [230, 231], CO [206], H2O[206], CHz [232], CH3 [232], CH4 [233], OCH3 [234-236], C3H5 [228, 229]) which are of catalytic interest. 

The initial heavy deposits of coke almost certainly originate mainly from asphaltenes in the feed. Preferential adsorption of asphaltene fractions have been observed on cobalt and molybdenum oxides [23] and sulphides [24] as well as on sulphided nickel-molybdenum... 

For molybdenum oxide, for example, it is shown that the adsorption of 4% (in mass) of polyanihne is enough to enhance the electrode response, affecting the potential in which the formation of the so-called molybdenum blue is formed. For arsenic oxide, it is shown that the modification of the oxide surface by 13% polyaniline adsorption was responsible for enhancement of the oxide response to apphed potential as well as for a change in the potentials observed for oxidation-reduction processes occurring in the unmodified oxide. 

Adsorption of caffeine, dimethylglyoxime, and rodamin-B on lamellar molybdenum oxide... 

In this chapter, specific examples of the use ofMo03 as a molecular sieve, will be discussed the synthesis and characterization of M0O3 intercalation compounds with nicotinamide (nic) and hexamethylenetetramine (hmta) [20], as well the study of the adsorption of caffeine (call), dimethylglyoxime (dmg), and rodamin-B (rod-B) on lamellar molybdenum oxide will be presented. 

Oxygen-containing compounds such as alcohols also undergo dissociative chemisorption, an example being the adsorption of gaseous methanol on molybdenum oxide catalysts (Eq. 5-28). Such metal oxides, and in particular mixed metal oxides, act as redox catalysts, as we shall see in Section 5.3.3. 

The constants R and k obviously depend on the type of base material used, with molybdenum-containing stainless steel 316 SS having other values than the normally used 304 SS. To explain these differences, it was assumed that molybdenum ions present in the spinel oxide film are oxidized under normal water chemistry conditions, creating additional vacancies which may act as additional adsorption sites for cobalt and other transition metal ions. Under hydrogen water chemistry conditions, the difference in Co buildup rate between these two base materials vanishes, obviously due to the fact that no molybdenum oxidation occurs so that no additional vacancies in the oxide film are formed. 

Matsuda, S, Kamo, T. Imahashi, J.. and Nakajima, F., Adsorption and oxidative desorption of hydrogen sulfide by molybdenum oxide-titanium dioxide, Ind, Eng. Chem. Fund.. 21(1). 18-22 (1982). 

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