Continuous-flow reactor setup for operando x-ray absorption spectroscopy of high pressure heterogeneous liquid–solid catalytic processes

A continuous-flow reactor and a continuous-flow setup compatible with operando x-ray absorption spectroscopy (XAS) were designed for safely studying liquid-phase reactions on solid high atomic number transition metal catalysts (e.g., Au, Pd, and Pt) under pressures up to 100 bars with temperatures up to 100 °C. The reactor has a stainless-steel body, 2 mm thick polyether ether ketone (PEEK) x-ray windows, and a low internal volume of 0.31 ml. The rectangular chamber (6 × 5 × 1 mm³) between the PEEK x-ray windows allows us to perform XAS studies of packed beds or monoliths in the transmission mode at any position in the cell over a length of 60 mm. A 146° wide-angle beam access also allows recording complementary x-ray fluorescence or x-ray diffraction signals. The setup was engineered to continuously feed a single-phase liquid flow saturated with one or more gaseous reactants to the liquid–solid XAS reactor containing no free gas phase for enhanced process safety and sample homogeneity. The proof of concept for the continuous-flow XAS cell and high-pressure setup was provided by operando XAS measurements during the direct synthesis of hydrogen peroxide at room temperature and 40 bars using a 35 ± 5 mg catalyst (1 wt. % Pd/TiO₂) and inline near-infrared spectroscopy. The experiments prove that the system is well suited to follow the reaction in the liquid phase while recording high-quality XAS data, paving the way for detailed studies on the catalyst structure and structure–activity relationships.