Continuous synthesis of Cu/ZnO/Al2O3 nanoparticles in a co-precipitation reaction using a silicon based microfluidic reactor

Abstract

CuO/ZnO/Al₂O₃ catalysts were continuously synthesized in a microfluidic reactor, analyzed by X-ray diffraction (XRD), physisorption (BET), chemisorption, electron microscopy and X-ray absorption spectroscopy (XAS), and tested for methanol synthesis from CO-rich synthesis gas. The results were compared to those obtained from CuO/ZnO and CuO/ZnO/Al₂O₃ produced by conventional co-precipitation in a batch reactor. The predominant phase of the aged precursor from microfluidic co-precipitation was identified as zincian malachite. After calcination the microfluidically synthesized catalyst exhibited smaller CuO crystallites, a larger BET surface area, a rather uniform morphology and a homogeneous distribution of Cu and Zn compared to catalysts prepared by batch co-precipitation. H₂-Temperature programmed reduction (TPR) showed that Cu species in CuO/ZnO/Al₂O₃ from microfluidic co-precipitation were more easily reducible. In situ Cu and Zn K-edge XAS during the TPR indicated reduction of Cu²⁺ to Cu⁰ between 150 °C and 240 °C, without detectable reduction of Zn. N₂O pulse chemisorption evidenced an enlarged Cu surface area of the nanoparticles from the microfluidic synthesis. Based on activity tests in methanol synthesis, at 250 °C the microfluidically synthesized Cu/ZnO/Al₂O₃ catalysts showed better performance than the catalyst from batch preparation when 1 mol% CO₂ was present in the synthesis gas. Dimethyl ether formed as a side product. As the microreactor is specially designed for high X-ray transmission with a thin Si/glass observation window, this study opens interesting perspectives for investigating the formation of catalyst precursors at the early stage of precipitation in future.