Iridium clusters for catalytic partial oxidation of methane -: an in situ transmission and fluorescence XAFS study

In situ transmission and fluorescence EXAFS combined with on-line gas analysis have provided new insight in the structural changes and the catalytic properties of Ir clusters during the catalytic partial oxidation (CPO) of methane. A novel in situ fluorescence XAFS setup with a multi element silicon drift detector allows time-resolved in situ studies on catalysts with small noble metal concentrations. Significant structural differences were found between a 0.5 wt% Ir/Al2O3 and a 2.5 wt% Ir/Al2O3 catalyst upon treatment in He and H-2. After He treatment metallic clusters form on high loading Ir catalysts but not when the loading is small. Upon H-2 treatment metallic Ir clusters are detected in both catalysts, but the particle size is smaller when a low loading is used. The smaller clusters appear to be more sensitive to oxidation. The CPO reaction is found to ignite at 320 degreesC, nearly independent of the residence time and the Ir cluster size. The structure of the clusters changes significantly at the ignition point. Below the ignition point they are partly (2.5 wt% Ir) or nearly fully (0.5 wt% Ir) oxidized and above the ignition temperature they are abruptly reduced. The catalytic and structural changes are reversible.