In situ and operando study of catalysts during high-temperature high-pressure catalysis in a fixed-bed plug flow reactor with x-ray absorption spectroscopy.

Abstract

Numerous important catalytic reactions, such as Fischer-Tropsch synthesis (FTS), are performed under harsh conditions in terms of high temperature of a catalyst in a mixture of reactants at a high pressure. There has been a lack of an intrinsic correlation between a catalytic performance and its corresponding catalyst structure due to the unavailable information on the authentic structure of the catalyst during catalysis under a high-temperature high-pressure (HTHP) condition. Here, we report in situ/operando studies of Co catalysts during catalysis under HTHP conditions using x-ray absorption spectroscopy (XAS). A high-temperature high-pressure catalysis-XAS (HTHP Catalysis-XAS) system using a thin, small quartz or beryllium tube as the reactor was built for in situ/operando characterization of high-energy absorption edges of 4d transition metals or low-energy absorption edges of 3d/4d transition metals under high-temperature high-pressure conditions, respectively. This reactor can be used for HTHP catalysis performed at a temperature of up to 550 °C and a gas pressure of up to 60 bars for uncovering the chemical states and coordination environments of metal atoms of these catalysts during HTHP catalysis. The capability of collecting XAS data during HTHP catalysis was confirmed through tests at 400oC in the mixture of 20 bar mixture of reactants at beamline endstation. The operando studies of Ru catalyst particles under Fischer-Tropsch catalytic conditions with extended x-ray absorption fine structure spectroscopy revealed a restructuring of the Ru catalyst at 250 °C in the mixture of 6 bars CO and 12 bars H2 during FTS (30 ml/min), which was not observed at 300 °C in 1bar H2 (20 ml/min). This observation suggests new chemistry for metal catalysts under HTHP condition inaccessible due to a lack of applicable characterizations. These tests confirmed the function of this HTHP Catalysis-XAS system for in situ/operando characterizations of catalysts during HTHP catalysis.