Promises for Ultrasonic Waves on Activity of Silica Gel and Some Supported Catalysts

Abstract

The effect of various types of irradiation, such as «-particles, neutrons, and -rays on the activity of catalysts (Coekelbergs et al., 1962;Mikovsky and Weisz, 1962;Taylor, 1965; Taylor, 1968; Weisz and Swegler, 1955) is reported. In these studies the catalytic activity increased significantly with the radiation treatment. In all cases the commercially available catalyst was irradiated. Usually, fairly large dosages of irradiation were required to produce significant changes in the catalytic activity (Mikovsky and Weisz, 1962).Recently, another type of irradiation, ultrasonics, was used to increase the activity of a catalyst (Li et al., 1964; Ranganathan et al, 1971). The ultrasonic treatment (insonation) was carried out during the manufacture of the catalyst as opposed to the irradiation of the commercially available catalyst. The information reported in the literature is as yet too scant to afford a clear picture of the subject. The situation is made complex by the several variables in the case of irradiation with ultrasonics-the frequency, intensity, duration of insonation, and the atmosphere over the catalyst system during irradiation.The literature on insonation of catalysts can be divided in three categories: (I) insonation of the catalyst during the preparation stage; (II) insonation of the already prepared catalyst; and (III) insonation of the reaction mixture (including the catalyst). Two of these systems have been discussed in detail in one of our earlier papers (Ranganathan et al., 1971). A brief introduction is in order. Case I: Insonation of Catalyst during Preparation StageThe catalyst is insonated during the preparation stage. As the ultrasonics produce cavitation and mixing at the microlevel, insonation affects the catalyst particle size, crystallite size, pore size, and the pore-size distribution (Kapustin, 1962;Li et al., 1964;Slaczka, 1964).

Sonochemistry: Past, Present and Future

“…The use of ultrasound during the precipitation of a mixed chromium-molybdenum catalyst for the oxidation of metha-no I to formaldehyde 54 gave catalyst with increased specific area, which led to increased catalytic activity. But ifthe ultrasound is applied after precipitation, it was led to agglomeration, surface area reduction and loss of activity 55 . Ranganathan et al 56 reported that sonication for one hour of aqueous suspensions of various metal oxide (Cr 2 0 3 , Mn0 2 , Co 2 0 3 ) and alumina prior to drying and calcination yielded catalyst with higher dispersity of metal oxide and higher activity in the decomposition of hydrogen peroxide.…”

A Sonoquímica

“…Contudo, o uso dos ultra-sons para facilitar a impregnação de reagentes em suportes sólidos não é novo, existindo referências na literatura desde o início da década de 70. Nesta época, Ranganathan [34] mostrou que a irradiação, por 1 hora, de suspensões aquosas de vários óxidos metálicos (Cr203. Mn02 e Co203) e alumina, antes da secagem e da calcinação, produzia catalisadores com um alto grau de dispersão metálica e uma grande actividade na decomposição do peróxido de hidrogénio.…”