Surface Areas of Pt–Rh Catalyst Gauzes Used for Ammonia Oxidation

Abstract

The changes in structure and surface composition of new and used Pt–Rh catalyst gauzes have been studied by gas adsorption methods. The total surface area was measured by physical adsorption of Kr (Kr-BET). The surface area of used samples varied between 0.01 and 0.06 m2/g representing an increase in surface area of 2 to 20 times compared with the surface area of new catalysts. A gauze with Rh2O3whiskers exhibited the largest increase in surface area. Except for this abnormal sample, the top gauze in the catalyst pack exhibited the largest surface area. The active surface area of the Pt–Rh gauze catalysts was measured by H2-titration/adsorption. It was assumed that Pt represents the active part of the catalyst. When exposing a spent gauze to H2at 298 K, only platinum oxide on the catalyst surface was titrated. During operation the catalyst deactivates partly by the formation of Rh2O3, which was found to be inert against H2at 298 K. However, reduction of used catalyst gauze in H2at 623 K and reoxidation at 623 K made all the Rh2O3reactive toward H2at 298 K, indicating the presence of two forms of Rh2O3. It was observed that only the first gauze in a gauze pack from a NH3-oxidation reactor comprised a noteworthy active area. All the other gauzes investigated were more or less blocked by Rh2O3.