Monolayer catalysts

Similarly to the Pt monolayer catalysts, a series of Pd monolayers deposited on different metal single crystals were tested for ORR activity. The results are shown in Fig. 9.22. The ORR activity increases in the order PdML/Ru(0001) < PdML/fr(lll) < PdMi,/ Rh(lll)[Pg.299]

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

Monolayer coverage of vanadium oxide on tin oxide support was determined by a simple method of low temperature oxygen chemisorption and was supported by solid-state NMR and ESR techniques. These results clearly indicate the completion of a monolayer formation at about 3.2 wt.% V2O5 on tin oxide support (30 m g" surface area). The oxygen uptake capacity of the catalysts directly correlates with their catalytic activity for the partial oxidation of methanol confirming that the sites responsible for oxygen chemisorption and oxidation activity are one and the same. The monolayer catalysts are the best partial oxidation catalysts. 

Figure 6.21. Schematics of currently pursued Pt-based electrocatalyst concepts for the ORR. (A) Pt bulk alloys (B) Pt alloy monolayer catalyst concepts (C) Pt skin catalyst concept (D) De-alloyed Pt core-shell catalyst concept.

The Pt alloy monolayer nanoparticle catalysts (e.g., Pt-Re layer on Pd cores) showed a clearly improved specific (Pt surface normalized) ORR activity their Pt mass-based electrocatalytic activity, however, exceeded that of pure Pt catalysts by an impressive factor of 18 x— 20 x. Their noble metal (Pt, Re, and Pd) mass-based activity improvement was still about a factor 4x. The Tafel slope in the 800-950 mV/RHE range suggested that the surface accumulation of Pt-OH species is delayed on the Pt monolayer catalyst. The enormous increase in Pt mass-based activity is obviously due to the small amount of Pt metal inside the Pt monolayer. 

Pt alloy monolayer catalysts exhibited even more active ORR behavior compared to Pt monolayer catalysts. To understand this phenomenon computational DFT studies were carried out. The hypothesis to be tested was that, for instance, Ru metal atoms in the Pt—Ru monolayer are OH-covered and could inhibit the adsorption of additional OH on neighboring surface sites (adsorbate-adsorbate repulsion effect). A very similar hypothesis was put forward about three years earlier by Paulus et al. [105] who postulated that Co surface atoms might exhibit a so-called common-ion effect, that is, they could repel like species from neighboring sites. A combined computational-experimental study finally confirmed this hypothesis [123] If oxophilic atoms such as Ru or Os were incorporated into the Pt monolayer catalysts, the formation of adjacent surface OH was delayed, if not inhibited. Oxo-phobic atoms, such as Au, displayed the opposite effect, would not inhibit Pt—OH formation, and were found to be detrimental to the overall ORR activity. 

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

Catalyst Structure C in Figure 6.21 is commonly referred to as the Pt-skin catalysts [87,95,107,124,125]. The term Pt skin catalysts will here be used to refer to a monolayer of pure Pt sitting on a Pt-depleted Pt alloy core in contrast, a Pt monolayer catalyst was referred above to as a monolayer of pure Pt on top of... 

Specific kinetic effect. This would apply to the sulfided monolayer catalyst. Cobalt may affect adsorption-desorption properties or intrinsic activity of vacancies. No definitive data exist to support this proposal. 

M. C., Moulijn, J.A., Medema, J., de Beer, V.H.J. and Gellings, P.J. (1980) Vanadium oxide monolayer catalysts. 3. A Raman spectroscopic and temperature-programmed reduction study of monolayer and crystal-type vanadia on various supports. J. Phys. Chem., 84, 2783. 

In the preceding sections the use of catalysts in which vanadium oxides are supported on a more or less inert carrier has been mentioned quite often. Because of the importance of this type of catalyst they are discussed more extensively in this section. Often a distinction is made between the normal supported catalysts and so called monolayer catalysts. In the latter the vanadium oxide is supposed to be dispersed in a monomolecular layer on the support, which may be covered completely or only partly. The normal supported catalysts are usually made by impregnation, either wet or dry, of the porous carrier with an aqueous solution, often of NH4V03, sometimes with oxalate added.12 14,75,95,139,140... 

Bond GC, Tahir SF. Vanadium oxide monolayer catalysts Preparation, characterization and catalytic activity. Applied Catalysis. 1991 71(1) 1—31. 

TABLE 1 Catalytic Activity of a Monolayer Catalyst of Molybdenum and Vanadium Oxides on Alumina with a Mo V Atomic Ratio of 1 1 for Propane ODH (Banares and Khatib, 2004), Measured in a Conventional Fixed-Bed Reactor and a Fixed-Bed Reactor Cell for Raman spectroscopy. 

Bond and Flamerz Tahir [62] reviewed the literature on the preparation, structure and catalytic properties of vanadium oxide monolayer catalysts. This catalyst plays an essential role in the selective catalytic reduction of NOx-... 

The concept of a Pt monolayer catalyst was first verified with a Pt submonolayer on Ru substrate. This approach radically changed the design of the Pt-Ru catalysts and it is likely to similarly affect a broad range of catalysts. It facilitates an ultimate reduction of Pt loadings in Pt-Ru catalysts by depositing Pt only at the surface of Ru nanoparticles, so that the most of the Pt atoms become available for the catalytic reaction. Ru (10%) nanoparticles on Vulcan XC-72 carbon were heated in an H2 atmosphere at 3()0°C for 2 h. This temperature is much lower than that required for bulk Ru... 

A new method of synthesis of selective platinum catalysts for the hydrogenation of unsaturated carbonyl compounds is presented. Platinum was deposited on the supports tailored with the monolayer of transition metal oxide. Selectivity of these catalysts strongly depended on the type of inorganic support as well as on the type of transition metal in the monolayer. Catalysts were tested in the hydrogenation of furfural, crotonaldehyde and cinnamaldehyde. Selectivity of the synthesis of the appropriate unsaturated alcohols was enhanced when compared with the reactions performed over classical Pt-metal oxide catalysts. 

Vanadia on titania catalysts prepared by wet impregnation (ammonium metavanadate) and monolayer catalysts prepared by grafting using vanadyl acetylacet-onate were compared [20]. It was demonstrated that monolayer catalysts show better activities at similar vanadium loadings than those of commercial catalysts. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

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