One-pot microwave synthesis of Pd modified titanium dioxide nanocrystals for 3D aerogel monoliths with efficient visible-light photocatalytic activity in a heated gas flow reactor

Abstract

Harvesting solar energy and efficiently converting it into a chemical energy carrier like hydrogen (H₂) is an important topic in photocatalysis. Here, we present a fast and simple approach for the one-pot synthesis of crystalline semiconductor nanoparticles modified with co-catalysts that can be used as nanobuilding blocks for nanoparticle-based aerogels with excellent visible-light photocatalytic activity. The microwave-assisted nonaqueous sol–gel method allows titanium dioxide (TiO₂) nanoparticles to be synthesized and modified with palladium (Pd) ions and Pd metal nanoparticles in a single step. Their assembly into aerogel monoliths preserves the modified properties of the TiO₂ nanobuilding blocks, resulting in morphological properties that are advantageous for the photocatalytic H₂ production from methanol (CH₃OH) oxidation. By controlling the amount of Pd doping and Pd nanoparticle loading, the nanoparticle-based aerogels showed significantly improved photoexcited charge generation and separation efficiency under visible light. In addition, we present a novel reactor design specifically developed for 3D aerogel monoliths that allows control of light intensity, gas flow, reactant concentration, and temperature, enabling the study of all key experimental parameters to optimize photocatalytic H₂ production. The visible light absorbed by the aerogels was found to be the driving force behind the efficient photocatalytic activity. Our Pd modified TiO₂ nanoparticle-based aerogels achieved H₂ production rate of 117.5 mmol g⁻¹ h⁻¹ with good stability for 3 days under visible light thanks to the prevention of carbon monoxide (CO) poisoning. The simultaneous optimization of the material composition and the matching photoreactor form the decisive basis for getting the most out of monolithic 3D photocatalysts.