Scaling heterogeneous catalysts down to the nanoscale enhances surface area to volume ratios, maximizes active sites and elicits functional properties commensurate with particle size. In order to maintain the few-nm size domain by prevention of Ostwald ripening, reduce the use of bulk rare/noble metals which typically comprise nanocatalysts, and enhance separability and recovery, catalyst nanoparticles are typically supported on larger nano or microparticles. Often, the catalyst and support can behave in a synergistic manner leading to enhanced activity. Further manipulation of supported nanoparticle systems in order to modulate catalyst performance is most commonly achieved through the addition of organic molecules to form self-assembled monolayers (SAMs). However, such organic moieties are vulnerable to reaction environment. We report the sustainable surfactant free synthesis of low-loading Pd nanoparticles on magnetically separable Fe2O3 nanostructured supports. The particles are utilized for the aqueous reduction of nitroaromatics and common dyes to their respective amines. These reaction systems are of intense current interest with broad reaching implications for organic synthesis, environmental remediation and as common model reaction systems for nanocatalyst development. Interestingly, both Fe2O3 and Pd are active catalysts for such reaction and the combination thereof has been shown to be synergistic. We saught to modify these particles in a non-destructive but permanent manner to study the outcomes on catalyst performance. Utilizing atomic layer deposition (ALD), we dosed a sparse amount of ZnO; a material known to be inactive for these chemical transformations. We postulate that ZnO nucleated and grew preferentially on Fe2O3 surface defects and naturally occuring hydroxyl surface termini, leaving the Pd particles uncoated. The overall nanocatalyst structure appears unchanged, but the catalytic activity in some cases has been successfuly altered. The observed dampening of Fe2O3 influence highlights the degree of synergistic effect of the catalyst-support system and the potential to modulate catalyst activity by non-destructive and selective ALD post-modification, which is inherently more robust than organic SAMs.

Modifying the activity of low-loading Pd@Fe2O3 nanocatalysts by sparse ZnO atomic layer deposition

ACS Fall 2021

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