Flame spray pyrolysis makes highly loaded Cu nanoparticles on ZrO₂ for CO₂-to-methanol hydrogenation

This paper deals with CuO/ZrO₂ catalysts with extremely high Cu loading and their catalytic activity for CO₂ hydrogenation to methanol. Because of aiming an industrial application, we chose a flame spray pyrolysis (FSP) technique as a simple and rapid catalyst preparation method. Thanks to the FSP, we succeeded to prepare 20–80 wt% CuO/ZrO₂ catalysts. Interestingly, the catalyst structure changed with the Cu loading. In the case of Cu loading = 20 wt%, CuO nanoparticles (ca. 5 nm) were supported on tetragonal ZrO₂ particles (5–10 nm), observed by high-angle annular dark-field scanning transmission electron microscopy. Of note, the catalyst with 60 wt% of Cu was ZrO₂@CuO core-shell nanoparticles: ZrO₂ aggregates were covered with many CuO nanoparticles (<5 nm). When the Cu loading was 80 wt%, crystalline CuO particles (ca. 10 nm) as well as CuO nanoparticles (<5 nm) were supported on the above ZrO₂ aggregates. The catalysts reduced by H₂ at 300 °C consisted of Cu nanoparticles (<20 nm) and ZrO₂ nanoparticles (5–10 nm). With decreasing the Cu loading, the interaction between the Cu and the ZrO₂ became strong. The strong interaction caused high selectivity to methanol. In contrast to 20 wt_Cu% CuO/ZrO₂, 80 wt_Cu% CuO/ZrO₂ contained a large number of active sites for CO₂ conversion, while the interaction between Cu and ZrO₂ was weak. Therefore, the catalyst exhibited high yield and low selectivity to methanol. Among the prepared catalysts, at Cu loading = 60 wt%, the catalytic performance was better than that of a commercial CuO/ZnO/Al₂O₃. This is because the catalyst combined the advantages of both the 20 wt% CuO/ZrO₂ (Cu-ZrO₂ interaction) and the 80 wt% CuO/ZrO₂ (a large number of active sites).