Surfactant-free colloidal syntheses of precious metal nanoparticles for improved catalysts

Colloidal syntheses of nanomaterials offer multiple benefits to study, understand and optimize un-supported and supported catalysts. In particular, colloidal syntheses are relevant to synthetize (precious) metal nanoparticles. By separating the synthesis of the active phase nanoparticles from supporting steps, a deeper knowledge and a rational control on supported catalyst properties is gained. The effect of nanoparticle size, shape, composition, nature of support or metal loading on a support can be studied in more systematic ways. The fundamental knowledge gained paves the way for catalyst optimization by tuning the catalyst activity, selectivity, and stability. However, a major drawback is that most colloidal syntheses require the use of additives or surfactants, which are detrimental to most catalytic reactions since they typically block catalyst active sites. Surfactant removal typically adds complexity, can introduce a lack of reproducibility, is energy consuming, generates waste, and prevents the full exploitation of the many benefits of colloidal syntheses for catalysis. Several surfactant-free strategies to obtain stable colloidal nanoparticles are here reviewed. A focus is given to laser synthesis and processing of colloids (LSPC), solution plasma process (SPP), N,N-dimethylformamide (DMF), polyols, and recently reported mono-alcohols based syntheses. The relevance of these synthetic approaches for catalysis is detailed with a focus on heterogeneous catalysis and electro-catalysis.