Stability of Pd-Pt catalysts in low-temperature wet methane combustion: Metal ratio and particle reconstruction

Palladium-platinum bimetallic catalysts are known for high activity in methane combustion, with phase transformations and structural changes occurring in a high-temperature oxidative atmosphere. Previous research has typically dealt with reaction temperatures exceeding those of the natural gas vehicle (NGV) exhaust, especially for lean burn engines, and often in the absence of water. This study evaluates the effect of the Pd:Pt ratio (from 5:1, 4:1, ... to 1:4, 1:5) on the Pd-Pt/gamma-Al2O3 catalyst stability during and after 40-h in situ hydrothermal ageing at 400-550 degrees C (5% water). Pt presence at Pd-Pt 1:1 to 4:1 ratios is found to be optimal from the viewpoint of activity, with the most stable formulation being a 1:1 Pd-Pt catalyst. Platinum excess above a 1:1 ratio suppresses deactivation at 400 degrees C, but at lower activity levels. The effect is not governed significantly by the particle size but by the ratio of Pd: Pt. The comparative experiments performed with Pd core-Pt shell nanoparticles, co-deposited monometallic particles and monometallic mixed supported catalysts, along with EDX mapping of the used catalysts, suggest significant structural changes when such particles progressively transform into alloyed structures, with activity and stability approaching those of the alloyed nanoparticles. The results suggest that platinum vaporization is significant in the wet feed at low surface oxygen concentrations, even at the temperature interval of 400-550 degrees C. It appears that the proper Pd-Pt metal ratio has a governing effect on the activity and stable behaviour in wet low-temperature methane combustion rather than the catalyst preparation method. (C) 2018 Elsevier Ltd. All rights reserved.