Oxygen, chemisorption catalysts

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. 

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. 

Oxygen chemisorption measurements were performed in the above flow apparatus using He as carrier gas (30 Ncm -min" ). Prior to chemisorption measurements, catalyst samples (0.25 - 1.00 g) were treated "in situ" for 15 min in a flow of CH4/02/He (Pch4 02 He 2 1 7) reaction mixture at 550-... 

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. 

Vanadium Catalysts Supported on Titanium (Anatase) Characterized by Ammonia and Low-Temperature Oxygen Chemisorption... 

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. 

A similar relationship can be observed with promoted M0S2. Each family of catalysts has its own linear correlation, which cannot be compared to each other directly because of the corrosivity problem. More recently, low-temperature oxygen chemisorption has been claimed to be more reliable, but it also lacks a well-determined stoichiometry (52). Oxygen chemisorption has also been applied to tungsten and rhenium sulfides, as well as promoted molybdenum and tungsten sulfides. In the isotropic class, it has been applied only to ruthenium sulfide, in which case it gives approximately the same result as a BET measurement due to the isotropic nature of this sulfide (41). 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

The experimental apparatus and the silver catalyst preparation and characterization procedure is described in detail elsewhere (10). The porous catalyst film had a superficial surface area of 2 cm2 and could adsorb approximately (2 +. 5) 10-b moles O2 as determined by oxygen chemisorption followed by titration with ethylene (10). The reactor had a volume of 30 cm3and over the range of flowrates used behaved as a well mixed reactor (10, 11). Further experimental details are given in references (10) and (11). 

The morphology and surface structure of molybdenum sulfide on two commercial HDN catalysts have been examined. Transmission electron microscopy results indicated that the M0S2 stack length increased and the stack density decreased after commercial use. The changes of the amount of low temperature oxygen chemisorption on HDN catalyst samples showed that the destruction of the microstructure of Mo species took place during the reaction. The reasons of the HDN catalyst deactivation have been discussed. 

Figure 3.3.11 Computational volcano curve of the ORR activity of well-defined metal surfaces versus their oxygen chemisorption energy, A/i0. Metals on the left bind oxygen too strongly, resulting in low ORR activity metals on the right bind too weakly, also resulting in low ORR rates. Pt is the most active monometallic ORR catalyst. The top of the volcano curve represents an unknown optimal catalyst and can be achieved by lowering the O chemisorption energy of Pt somewhat (arrow and red dotted lines). Figure adapted from [15].

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