2D-mapping of the catalyst structure inside a catalytic microreactor at work: partial oxidation of methane over Rh/Al2O3.

Tremendous changes of the structure of Rh particles occurred during partial methane oxidation to hydrogen and carbon monoxide over a 2.5 wt % Rh/Al(2)O(3) catalyst upon ignition of the catalytic reaction. Furthermore, near the ignition temperature a variation in the Rh-valence state along the catalyst bed was observed. By combining hard X-ray absorption spectroscopy (X-ray absorption near edge structure, XANES) with a charged coupled device (CCD) camera and using a suitable spectroscopic cell with gas supply and on-line mass spectrometry, we demonstrate that 2D-mapping of the Rh-oxidation state in a catalyst bed can be achieved during the catalytic reaction. For this purpose, X-ray images were recorded with the CCD camera at each energy around the Rh K-edge with and without the spectroscopic cell. This resulted effectively in the transmitted and incident intensity at each energy and at each pixel of the spectroscopic cell. Reconstruction of the full Rh K-edge XANES spectra at each pixel revealed the local distribution of oxidized and metallic Rh-species in the catalyst bed. Along the catalyst bed, structural changes were found with a steep gradient within less than 100 microm. Furthermore, a characteristic cone toward the inlet of the spectroscopic cell was observed.