Deactivation of palladium catalyst in catalytic combustion of methane

Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NOx emissions. Palladium-on-stabilised alumina has been found t o be the most efficient catalyst for the complete oxidation of methane to c arbon dioxide and water. However, its poor durability is considered to be a n obstruction for the development of catalytic combustion. This work was ai med at identifying the origin of this deactivation: metal sintering, suppor t sintering, transformation PdO --> Pd + 1/2O(2) or coking. Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm lengt h lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb su bstrates. Experiments were performed at GHSV of 50 000 h(-1) in lab test an d 500 000 h(-1) in pilot test. The catalysts used were palladium on differe nt supports on cordierite substrate. The catalysts were characterised by XR D, STEM, ATG and XPS. In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conv ersion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support o n the catalyst deactivation was also investigated. It has been shown that s ome supports can surprisingly eliminate this oscillating behaviour. However , in pilot test, deactivation was found to be very rapid, even with stabili sed alumina supports. Furthermore, successive tests performed on the same c atalyst revealed that the activity (light-off temperature, conversion) fall s strongly from one test to another. Then, the stabilised alumina support was calcined at 1230 degrees C for 16 h prior to its impregnation by palladium, in order to rule out its sinterin g. Experiments carried out on precalcined catalysts point out that deactiva tion is mostly correlated to the metal transformation under reaction condit ions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium. (C) 1999 Elsevier Science B.V. All rights reserved.