Silver Nanoparticles as thermal and Plasmon-enhanced Catalysts for the selective Hydrogenation of Carbonyls
Ag NPs are less active than Pt series metal NPs for the hydrogenation because of their d orbital structure, yet they feature a unique ability to selectively hydrogenate carbonyls while leaving C-C double and triple bonds untouched. We devised a rapid and simple synthesis of Ag and Fe2O3 NPs embedded into a carboxymethyl cellulose matrix, using microwave irradiation, which proved an excellent, selective, carbonyl reduction catalyst under 40 bar H2 (Li, ACS Sustainable Chem. Eng. 2016). This system was easy to make, recyclable and with limited leaching. In an effort to lower the high pressure requirement of this system, we explored the coupling of this reaction with visible light via activation of the surface plasmon band (SPR) of Ag NPs. We showed for the first time that Ag NPs could activate hydrogen and hydrogenate ketones and aldehydes under H2 atmospheric pressure, via irradiation of their SPR (Landry, ACS Catal. 2017). Laser light was used to probe specifically SPR and non-SPR wavelengths to prove without ambiguity the plasmon enhancement, and a hot electron mechanism was proposed. Compared to other catalytic systems, the plasmonically activated catalyst provides access to primary and secondary alcohols using milder conditions, in a highly atom economical fashion. We also used the same catalyst to afford an SPR-activated aldehyde to carboxylic acid oxidation reaction. My interest for plasmon-active NPs roots back to my PhD when I wrote a highly cited (>500 citations) tutorial review on principles underlying the SPR.