Laser ultrasound generation.
Laser ultrasound generation
Catalysts are ubiquitous in modern life, from enabling the efficient production of petroleum / plastics products to acting as catalytic converters in the reduction of climate-altering automobile emissions. Catalysts are often designed to be highly refractory, so that they survive in the operating conditions necessary for use for an economically-viable period of time before becoming “spent”, poisoned or otherwise inactive or insufficiently efficient. The refractory properties of the catalyst confer inherent significant resistance to breakdown which in turn makes it very difficult to recover the transition metal from the bulk catalyst at the end of life point. Ultrasound-assisted dissolution in strong acids is a current preferred mechanism.
It is known that the absorption of pulsed laser radiation in a thin layer of absorbing material creates a violent shock wave at very high frequencies up to the GHz-scale, and where the frequency and amplitude of the laser-generated shockwaves can be varied by varying the laser wavelength, pulse energy and pulse width, and the nature of the absorber.
John recently developed a new very short pulse Q-switched Nd:YAG laser operating at 1064 and 532 nm and a multi-chamber system for in-line production of nanoparticles. This same continuous flow reactor can be used for continuous flow catalyst reduction. We found that an aqueous slurry of spent platinum-based silica-supported catalyst could be efficiently pulverized into a form suitable for subsequent mild acid dissolution resulting in the creation of hexa-chloro-platinate ions. An added interesting feature of the process was the creation of a very fine silica particulate slurry which might find applications in chromatography or other materials science fields.
This laser-generated ultrasound technique is also under investigation for large-scale aqueous phase cracking of the PFAS (perfluoro-alkyl sulfonate) components of Aqueous Film Forming Foam (AFFF) fire-fighting systems. PFAS-family chemicals are extremely resistant to breakdown under normal conditions and as a result are accumulating in the environment and in the food-chain. There is an urgent need to find an environmentally respectful method of disposing of these chemicals or reducing them down to more tractable components.