The Effects of Ultrasound on Transition Metal Catalysts

Abstract

Investigation of a gas phase reaction suggested that the effects of ultrasound on heterogeneous catalysts were simply due to increased mass transport [239]. However, it has also been clearly shown that palladium and platinum blacks prepared by reduction of the metal salts in the presence of formaldehyde had up to 30% greater surface areas and showed increased activity in a number of representative reactions. These included the decomposition of water, the hydrogenation of hex-l-ene, and the oxidation of ethanol [240]. In addition, the catalysts showed increased paramagnetism and appeared to contain a higher concentration of atomic metal. Interestingly, Mal’tsev observed that increasing the frequency of the ultrasound used in the preparation of platinum blacks from 20 kHz to 3 MHz resulted in an increased level of catalytic activity. Conversely, the opposite trend has been observed in the case of the palladium blacks. There is no obvious explanation for this. A similar technique was used to prepare suspensions of platinum on silica gel. Hence, reduction of solutions of platinum complexes in the presence of ultrasound (440 kHz, 5 Wcm^-2) increased the available surface area of platinum by 80% with respect to a control sample. Further evidence has been provided by Cioffi and Prestegard who have suggested that modification of the catalyst surface may also be a factor to consider. Raney nickel was used to catalyse the incorporation of deuterium into a glycosphingolipid [242]. Catalytic deute- ration of non-reducing monosaccharides using this catalyst in D₂0 was demonstrated some time ago. However, the forcing conditions required restricted the general applicability of the method to simple saccharides. It was then shown that sonication of the substrate and deuterated Raney nickel in D₂0 gave unrearranged products in virtually quantitative yield. Moreover, the selectivity of the process appeared to be better than that observed under thermal conditions. The authors report that preliminary studies showed that similar results were obtained if the catalyst was exposed to ultrasound before addition of the substrate. This suggests that the rate enhancements observed may well owe more to modification of the catalyst surface than a simple increase in mass transport.