Uncovering the deactivation mechanism of Au catalyst with operando high spatial resolution IR and X-ray microspectroscopy measurements

Detecting the reaction mechanism of multistep catalytic transformations is essential for optimization of these complex processes. In this study, the mechanism of catalyst deactivation within a flow reactor was studied under reaction conditions. Spectral mapping of the catalyst and the organic phase along a flow reactor were performed with micrometer-sized synchrotron-based X-ray and IR beams, respectively, with a spatial resolution of 15 μm. Heterogeneous Au catalyst was packed in a flow reactor and activated toward the cascade reaction of pyran formation. X-ray absorption microspectroscopy measurements revealed that the highly oxidized Au(III), which is the catalytically active species, was continuously reduced along the flow reactor. IR microspectroscopy measurements detected a direct correlation between the reduction of the Au catalyst and deactivation of the catalytic process. It was observed that within 1.5 mm from the reactor's inlet all the catalytic reactivity was quenched. Microspectroscopy measurements determined that the reduction of Au(III) was induced by nucleophilic attack of butanol, which is one of the reactants in this reaction. Slower deactivation rates were measured once the reactants concentration was decreased by an order of magnitude. Under these conditions the reaction path within the flow reactor was increased from 1.5 to 6 mm. These results demonstrate the molecular level understanding of reaction mechanism which can be achieved by high spatial resolution microspectroscopy measurements.