Talking to surfaces… and understanding their answers
2011-11-21
During operation the platinum based gauze catalysts undergo a massive reconstruction of their surface morphology. This process is attributed to local excessive heat from the ammonia oxidation reaction which causes an intensified migration of platinum and rhodium atoms out of the crystal lattice which deposit at surface regions with a much lower temperature.
Passing through the catalyst assembly the ammonia content in the process gas decreases and with it the extent of the thermal etching.
The gaseous species PtO2 and RhO2 are especially formed at defect sites (chemical- or catalytic etching) and decompose under formation of the so called cauliflower structure. The formation of this epitaxic agglomerates is induced by the temperature gradient between the gauze surface and much lower temperature of the feed gas. Passing through the catalyst pack the temperature of the process gas increases and the temperature gradient becomes negligible.
Thermal and chemical etching are dynamic processes accompanying the catalyst during it’s lifetime and causes it’s aging with the attendant circumstances of metal and activity losses.
The characteristic surface morphology of the single gauzes in the catalyst pack therefore illustrates the distinctive fingerprint of the individual plant with its particular operating conditions and operation mode.
The changes in structure and chemistry of the surface of a catalyst can be tracked using Umicore’s sophisticated analytical tools.
The analysis reveals what happened and enables us to determine the optimum balance between catalyst performance and requirements of the individual operating conditions of any particular plant.
