Spatial Changes of the Catalyst Structure inside a Fixed-Bed Microreactor during the Partial Oxidation of Methane

In situ X-ray absorption spectroscopy is a well-suited technique to gain information on the structure of heterogeneous catalysts. In certain cases a variation of the catalyst structure can occur inside a catalytic reactor as a result of temperature or concentration gradients. This requires spatially resolved molecular information on a microscale. Here, we have used a position sensitive X-ray camera to record XANES spectra of the catalytic reactor with a spatial resolution on the scale of a few micrometers [1]. Recently, we observed a strong spatial change of the oxidation state of rhodium in Rh-based catalysts during the partial oxidation of methane [1,2]. Both catalytic combustion and reforming (CCR) and a direct partial oxidation (DPO) mechanism have been proposed [3]. From experimental and modelling studies, it could be concluded that both the temperature and the flow (space velocity) influence the relative distribution of the gas composition during the catalytic partial oxidation of methane significantly. Hence, we investigated the influence of both parameters on the structural composition of the catalyst in the fixed-bed. Ideally, tomographic imaging could yield the three-dimensional distribution of the oxidation state inside the microreactor Tomography, however, requires the acquisition many images and thus much shorter acquisition times than the current ones. The experiments were performed at beamline X1 with a beam size of typically 3 mm x 1.5 mm and a Si(311