Metal oxides, catalysts temperature effect

CO oxidation on 1%Au supported on various metal oxide catalysts was carried out to determine the effect of metal oxide on the activity and stability of the catalysts during room temperature CO oxidation. Figure 4 shows the CO conversion as a function of time on stream on 1%Au supported on various metal oxides such as CO3O4, Fe Oj, NiO, ZrOj, and TiO. All the catalysts showed high initial CO conversions. The stability of the catalysts decreased in the following order TiO > ZrOj > NiO > FejOj > CO3O4. The stability of the catalysts appears to decrease with increasing basicity of the metal. 

Recently, there have been various studies of the effect of the adsorption temperature on the acidic properties of metal oxide catalysts. Tsutsumi and co-workers (84,85) studied calorimetrically the adsorption of ammonia and pyridine on H Y and NaY zeolites, silica-alumina, and silica between 313 and... 

Already in 1929 it was proposed by Schwab and Pietsch that the catalytic reaction on supported metal catalysts often takes place at the metal-oxide interface. This effect is known as adlineation, however, up to the present there is only little direct experimental evidence. In one example, the oxidation of CO on nanoscale gold, it is presently discussed whether the catalytic action takes place at the particle upport interface. Adlineation is strongly related to the effect of reverse spillover, where the effective pressure of the reactants in a catalytic process is enhanced by adsorption on the oxide material within the so-called collection zone and diffusion to the active metal particle (see Fig. 1.55 and also The Reactivity of Deposited Pd Clusters). The area of the collection zone and thus the reverse spillover are dependent on temperature, on the adsorption and diffusion properties of the reactants on the oxide material, as well as on the cluster density. 

One of the best ways of characterizing a supported catalyst is determination of dispersion and effective surface area of the catalyticaUy active component. The dispersion of metal oxide catalysts can be determined by selective chemisorption of oxygen at appropriate temperatures [14-16]. The dispersions obtained from oxygen chemisorption measurements on various catalysts are presented in table 1. The N2 BET surface areas of various samples are also shown in this table. As can be noted, dispersion for 20 wt% catalyst is similar, within experimental limitations, irrespective of their origin. The BET surface area measurements also reveal that both the preparation methods yield similar type of catalysts in terms of physico-chemical characteristics. These catalysts were further evaluated for selective oxidation of / -methox doluene to p-... 

The stop-effect, a drastic increase of the reaction rate when the feed concentration of a reactant is switched to zero, is observed for the catalytic dehydration of alcohols or the deamination of amines on aluminas, zeolites or more generally, on amphoteric metal oxide catalysts in the temperature range of 130 to 340°C [1, 2]. It can be described by different models, which make the hypothesis of different surface intermediates. Two basic models were discussed by Thullie and Renken [3] ... 

As to the reaction catalyst, Malinowski and Kehl, in 1960 to 1962, reported that the hydroxides of alkaline-earth metals, such as barium hydroxide, calcium hydroxide, magnesium hydroxide, strontium hydroxide, certain alkali-metal hydroxides, and some heavy-metal oxides are all effective as catalysts for the aldol reaction. Likewise, the hydroxides of tri- and tetra-valent rare-earth metals were shown to be active by Berlin and coworkers in the formation of formose under conditions of high temperature (110°) and pressure (1.8 atm.). Some organic bases, as well as certain inorganic bases, were also shown effective by Mizuno and co-... 

Nonetheless, rate expressions more complex than a simple power law are sometimes useful. For example, a power law expression does not provide any insight into the reasons for changing reactant order (i.e., a changing value of a ) with temperature or organic reactant concentration. However, such effects are frequently observed in oxidation reactions and are often consistent with more fundamentally based rate expressions. Consider, for example, what one would suppose to be the simple oxidation of methane. Golodets (p. 445) states that methane oxidation over metal oxide catalysts may be interpreted by the following mechanism ... 

---