Catalytic activity size-dependent

Ta(CH2Ph)5 to ethane or hydrogen [48]. EXAFS spectroscopy confirmed the existence of a Ta-Ta species, demonstrating the formation of tantalum clusters on silica. This supported that Ta sample catalyzed the metathesis of ethane at high temperature (250 "C) [49]. The initial conversion of ethane metathesis was 24% the molar ratio of methane to propane exceeded 1, and small quantities of butanes were also observed. This catalytic system was also found to convert propane into ethane and butanes [48, 50, 51]. The authors found that the observed catalytic activity was dependent on average cluster size. Increasing the average cluster diameter by 0.2-0.3 nm led to a decrease in activity [51]. 

Specifications and Packaging. Aluminum chloride s catalytic activity depends on its purity and particle size. Moisture contamination is an important concern and exposure to humid air must be prevented to preserve product integrity. Moisture contamination can be deterrnined by a sample s nonvolatile material content. After subliming, the material remaining is principally nonvolatile aluminum oxide. Water contamination leads to a higher content of nonvolatile material. 

A classical issue in transition-metal catalysis is the dependence of catalytic activity on changes in the particle size of the metal clusters in the nanosize region [14]. 

Figure 31 shows that among metal oxide supports, TOF markedly changes depending on not only the kind of metal oxides but also on their size [98]. Especially, fine particles of Ce02 with mean diameter of 5 nm present the highest catalytic activity. On the other hand, Prati and her coworkers [31] reported that gold NPs supported on activated carbons are very active and selective in the liquid phase oxidation of various alcohols. 

Gold has been recently demonstrated to be active in many catalytic reactions as reviewed by Bond and Thompson [1], However, as highlighted by Haruta [2], many different parameters play a role in determining activity (i.e. particle size and shape, preparation methods, nature of support) sometimes making difficult the comparison of experimental results. For example, catalytic activity evaluated against different units showed different trends, showing convincingly that beside total surface area, some other size-dependent factor has to be involved. 

The activity of gold catalyst is normally strongly size dependent and the control as well as the narrowest possible distribution of particle size represent the main goal for the production of an active gold catalyst. From a catalytic point of view, several preparation methods have been proposed for obtaining highly dispersed gold catalyst, most of them derived from deposition-precipitation method proposed by Haruta et al. [3]. 

Oxygen reduction on both carbon- and titania-supported Pt particles is dependent on particle size. A deactivation of the catalytic activity is observed for decreasing particle size on both supports. In addition, there is no evidence of any activation of the Pt above that of bulk Pt on either support. 

CO electro-oxidation exhibits a strong particle size dependence on both carbon-and titania-supported Au catalysts a strong deactivation of the reaction is observed for particle sizes below about 3 nm. In the case of the titania supports, however, a distinct activation of the reaction is also evident. This manifests itself in a strong decrease in the overpotential for the reaction, and an increase in activity as the particle size decreases in the range 8-3 nm. The result is a maximum in the catalytic activity with particle size. 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

Improved Filtration Rate Filterability is an important powder catalyst physical property. Sometimes, it can become more important than the catalyst activity depending on the chemical process. When a simple reaction requires less reaction time, a slow filtration operation can slow down the whole process. From a practical point of view, an ideal catalyst not only should have good activity, but also it should have good filtration. From catalyst development point of view, one should consider the relationship between catalyst particle size and its distribution with its catalytic activity and filterability. Smaller catalyst particle size will have better activity but will generally result in slower filtration rate. A narrower particle size distribution with proper particle size will provide a better filtration rate and maintain good activity. 

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