Oxygen, chemisorption oxidation

Only the surface layers of the catalyst soHd ate generaHy thought to participate in the reaction (125,133). This implies that while the bulk of the catalyst may have an oxidation state of 4+ under reactor conditions, the oxidation state of the surface vanadium may be very different. It has been postulated that both V" " and V " oxidation states exist on the surface of the catalyst, the latter arising from oxygen chemisorption (133). Phosphoms enrichment is also observed at the surface of the catalyst (125,126). The exact role of this excess surface phosphoms is not weH understood, but it may play a role in active site isolation and consequently, the oxidation state of the surface vanadium. 

Figure 2.5 Dynamics of oxygen chemisorption at Cu(110) and Mg(0001) surfaces based on XPS and HREELS leading to oxidation of ammonia and imide chemisorbed species. The reactive oxygen is a transient state Cr(s). With ammonia-rich mixture Pathway 2 is dominant whereas for oxygen-rich mixtures Pathway 1 dominates at 295 K.

To probe the early stage of oxygen chemisorption, that is, prior to the onset of surface reconstruction and oxide formation and relevant to our coadsorption reactivity studies, there were obvious advantages for STM observations to be made at cryogenic temperatures. 

That carbon monoxide could be oxidised in a facile reaction at cryogenic temperature (100 K) was first established in 1987 by XPS at an aluminium surface.21 The participation of reactive oxygen transients O 1 (s) was central to the mechanism proposed, whereas the chemisorbed oxide O2 state present at 295 K was unreactive. This provided a further impetus for the transient concept that was suggested for the mechanism of the oxidation of ammonia at a magnesium surface (see Chapter 2). Of particular relevance, and of crucial significance, was Ertl s observation by STM in 1992 that oxygen chemisorption at Al(lll) resulted in kinetically hot adatoms (Figures 4.1 and 4.7). 

Fig. 21. Ethylene oxidation to ethylene oxide (O) and CO2 (O) over Pd-Ag alloy films. Magnetic susceptibility data ( ) (14%) and heats of oxygen chemisorption ( ) (.131) are also included (1S6).

The pyrolysis of wood, oxygen chemisorption and oxidation of wood chars were carried out in a computerized coupled TG-FTIR system containing Cahn-R-100 electric balance, DuPont Model 990 thermal analyzer and Nicolet MX-1 Fourier transform infrared spectrometer. All of these sequential processes are carried out within the thermal balance without interruption. 

Another way of investigating structure is through the classical method on metals of varying catalyst particle size. The key to this method is to measure active catalyst surface areas in order to determine changes in turnover rates with ensemble size. In recent years several chemisorption techniques have been developed to titrate surface metal centers on oxides (25). In this volume Rao and Narashimha and Reddy report on the use of oxygen chemisorption to characterize supported vanadium oxide. 

Surface Bond Energies Thermochemical data are very scant in the area of oxygen chemisorption (57). These data would be of great value for interpreting spectroscopic and kinetic data and for the analysis of reaction mechanisms. The vast majority of the available data are for low oxidation state systems (55). Although calorimetry offers a means for direct measurements, for analysis of reaction pathways it is necessary to have detailed values for many types of species (M-OH, MO-H, M-OR, M-R, M-O, M-H), and these are usually... 

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... 

Table 1 Oxygen Chemisorption and Methanol Oxidation Activity Results on Various V205/SnQ2 Catalysts...

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. 

In this communication, the results of a systematic study of ammonia chemisorption on V2O -Ti02 (anatase) catalysts of different vanadia loading is reported." Low temperature oxygen chemisorption is also utilized to determine the monolayer loading of 20 on Ti02 (anatase). Partial oxidation of methanol to formaldehyde is studied as a model reaction on these catalysts and the activities of the catalysts are correlated with NH and O2 uptakes. 

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