Synchrotron Radiation Effects on Catalytic Systems As Probed with a Combined In-Situ UV−Vis/XAFS Spectroscopic Setup

UV−vis spectroscopy was used in a combined in-situ UV−vis/XAFS spectroscopic setup to study the synchrotron radiation effect on aqueous homogeneous copper solutions. Two different systems were studied. In the first study, the focus was on a copper bipyridine-catalyzed oxidation of benzyl alcohol to benzaldehyde with 2,2,6,6- tetramethylpiperidinyl-1-oxy and base as cocatalysts. It was found that when the reaction mixture is exposed to the X-ray beam, the features present in the in-situ UV−vis spectrum develop differently compared to the situation when the reaction mixture is not exposed to the X-ray beam. Besides a temperature effect of the X-ray beam, both the UV−vis analysis and the XAFS analysis showed a reducing influence of the X-ray beam on the sample. To investigate this in more detail, we studied a series of dilute aqueous copper solutions from different precursor salts, viz., Cu(NO3)2·3H2O, CuSO4·5H2O, CuCl2, and CuBr2. It was found that the different aqueous copper solutions have different stabilities under the influence of the X-ray beam. Especially the solution from the CuCl2 precursor salt was found to be unstable and to be subjected to reduction. These examples illustrate the need for a second technique, such as in-situ UV−vis spectroscopy, to evaluate the effect of synchrotron radiation used to measure in-situ XAFS on catalytic systems.