Identification of the Active Species Generated from Supported Pd Catalysts in Heck Reactions: An in situ Quick Scanning EXAFS Investigation

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 degrees C was reached. At the same time, the conversion of PhBr started, and during the following active phase of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd-0 clusters with a size of similar to 2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr4](2-), [Pd2Br6](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.